Improve Select Zero

Look for consecutively aligned vertices, improves the detection of internal degenerates in a face respect to the former version.
Adjusted precision, which now can be adjusted in the Redo panel when no degenerate triangles are found.

Added Texel Density Unit Scale in preferences

Allows to adjust the system unit scale to a custom unit by a multiplier for the texel density, so it is shown and managed in that custom unit (e.g., pixels per centimeter when the system unit is meters); the setting can be found in the add-on preferences. Fixes #146

Merge pull request #105 from BenjaminSauder/uv_unwrap

Uv unwrap

Baking revamp, Texel Density revision

BAKING:
- Restructured code and implemented a Try -> Finally structure to prevent data loss in case of error during the bake process.
- Avoided Circular Dependency when the image-to-bake-on is present in the blend file and used in the node tree of the materials that take part in the bake process. Two methods were used:
A - If the existing image origin is a file: "making unique" the image nodes in the materials during the bake will do the trick, while preserving the path of the image in stock, because the bake is done on it. Delete the image created by "making unique" the nodes afterwards.
B - If the existing image origin is generated and only exist internally in the blend file: a new image have to be created to bake onto. After the bake, assign it to the image nodes of the baked materials that use the original image, and rename it after the original. Delete the previous image. Fixes #84
- Removed use of "temp_sets", duplicate baked objects materials to avoid restorations after relinks, discards.
- Optimization when baking custom modes (modes using a "material_loaded")
- Added option in preferences: "Device for baking" (CPU, GPU or Default)
- Added option in preferences: global "Bake Color Space".
- Selection mode is now restored.
- Baking can be now performed on the active object while in edit mode whether it is actually selected or not.
- Reverted Poll changes of last commit.
- Certain bake modes can now be performed without a material in the baked object.
- Faster and cleaner poll messages.
- Remove and unlink material_empty and material_loaded when the bake of all sets is finished for no more hanging materials in the blend file after a bake. As a downside, they have to be created/imported every single time in certain scenarios (custom bake modes like material_id, cavity, etc, and lowpoly meshes in high-to-low bakes that wouldn't have a material). It's fast, though.
- Bake mode "Element ID" no longer delivers repeated colors when baking multiple objects.
- Added samples property to "Combined".

TEXEL DENSITY:
- Added enum to select the Texture Dimensions to consider when getting or setting the Texel Density value: from an image present in a material for each object (default), a specified baking resolution (TexTools tab > Size), or from a list of common sizes.

New version 1.4.3 push

Several sets baking fix

There were unnecessary loops affecting deletion of temporary baking nodes and objects, only noticeable when baking several sets of objects.
Fixes #73

Baking overhaul

- Correct results on Normal, Curvature an Ambient Occlusion maps; original shaders textures details will be preserved from now on.
- Weird rendering problems with Progressive Refine activated that happens since Blender 2.90 are now avoided by deactivated it temporarely while baking. AO, Curvature, and any Normal bake results should no longer differ between 2.83 and 2.90+.
- Speed-up for all bake modes, thanks to no more store/restore of materials. This is critical for baking high-poly objects, performance gains should be more noticeable as the polycount goes higher.
- New bake modes: Base Color, Metallic and Emission.
- Selected to Active changed in 2.90, Cage extrusion and Max Ray Distance properties were splitted, and this is now reflected internally and in the addon UI. Both can coexist, contrary to logic, and it is worth experimenting tuning them both on the same bake.
- No more pile of unnecessary messages in console when selecting baked maps in the bake panel - sets section of the addon.
- Curvature bake and Sun Position: bad relationship between addons fixed.
- Smoothness renamed to Glossiness, a more common term in the PBR shading world.
- ao_legacy hasn't been working since 2.80 because it was intended to be used along the Blender Internal renderer, it has been delivering the same results than the base AO all this time. Therefore, it has been removed from the list of baking modes.

Several bake modes fixes

- New Material ID baking: assign a unique, distinct color to every material, instead of using the material slot number as color index. Limitations: Support for up to 256 materials; due to the list being persistent only during the current session, between sessions (close and open de file) the result would be different if the materials with users in the scene change (creations, deletions, renames).
- Vertex Colors layers present in the objects were overwritten in certain bake modes (Material ID, Element ID, Cavity, Dust, Paint Base, Selection), with the side effect of making some of these modes don't work themselves, generating straight black images.
- Roughness and Smoothness were not delivering correct results due to the objects materials being replaced by void materials before the bake would start.
- Diffuse, Roughness and Smoothness weren't working properly in multi-object bakes, materials other than the active object active material slot weren't baking.
- Now, when baking Diffuse, Roughness and Smoothness, support image nodes created during baking are deleted once the bake ends.
- Roughness and Smoothness baked images backgrounds set both to black: no more white background for Smoothness.

Extended support for multi-object UV editing

- Align, Sort, Crop, Fill and Select Similar now work with multiple objects while editing the UVs.
- Now, Select Overlap behaves like in previous versions: all but one of the overlapping islands are selected, making it posible to manually fix the overlapping by using this operator in conjunction with manually moving the selected islands.
- Further optimization of the 90º Rotate Island operator.
- Some cleanup and minor corrections in op_smoothing_uv_islands, op_select_islands_flipped, op_select_islands_outline, op_rectify, settings, __init__ and LICENSE.txt

Icon 2.9x series update by: activemotionpictures

DavidRivera
Blender Foundation CERTIFIED TRAINER
https://www.youtube.com/channel/UCjBTceQ-XFxKNDoGk805zig
He has made these amazing icons to bite the look of textools, to the new Blender 29x.

inspired by this design by : https://youtu.be/dBWk2lwzgWc