ImageLoader uses bg thread for processing

[jazzay]

Modifies ImageLoader's load function to use background threads for the image decoding work. This avoids blocking the main thread that is especially noticeable when loading many images at once.

This was modelled after the other loader implementations that also use threads.

[emilk]

Thanks for the PR!

In some cases you want the blocking behavior though (to avoid a frame of showing no image) so it would be better if this was an optional behavior.

Also, this won't work on Wasm

[jazzay]

fixed the cranky errors. Will look at making this optional and disabled for Wasm.