fix: stop alt loaders on main mediachanging to prevent append race

[brandonocasey]

Description

This is most noticeable right now on safari on the hevc fmp4 stream that we have for testing. You can reproduce it in main by seeking around and force changing the representation to one with another codec:

vhs.representations().forEach((r, i) => r.enabled(i ===49))
vhs.representations().forEach((r, i) => r.enabled(i ===53))
vhs.representations().forEach((r, i) => r.enabled(i ===20))

Usually it will fail with an append error, because we change the codec and then try to append old codec data. This happens because we previously waited until mediachange to stopLoaders for audioLoader but for the mainLoader we stop right away. This causes a race condition where old audio gets appended before the media has fully changed, but after codecs have. The fix here is to stop/pause all segment loaders on mediachanging

Why didn't we see it sooner

Its not an issue to append data of the same codec to a sourcebuffer even if it isn't relevant. That and we always reset when starting to load that data append never caused an issue. Now that we switch codecs such an append will throw an error.

[gesinger]

Would be good to add a test

[gesinger]

Should this use stopLoaders? And should onGroupChanged no longer stopLoaders?

[brandonocasey]

Originally I tried using stopLoaders but that caused an issue because we abort the current audio playlist loader request when starting playback even though we aren't changing audio playlists. I will check if mediachange should just stop the audio playlist loader, since the segmentLoader will be stopped.

[brandonocasey]

OK after testing out changing onGroupChanged to only stop the activePlaylistLoader is doesn't work. Loaders are started and stopped frequently during the start of playback and if we don't stop them in mediachange too it can cause playback to hang because the segment loader is trying to load, but there isn't an activePlaylistlistLoader.

[brandonocasey]

test added

[gkatsev]

Had a issue with safari but it may be better once all the other PRs are merged. Approving preliminarily and we'll be testing in main.