Merge some vanilla texture atlases so the game has to generate less of them?

[muzikbike]

Request Description

Following up on #2941 since this may also improve resource loading times and VRAM usage, though I don't know if implementing these changes might impact mod compatibility.

Currently the vanilla game generates fourteen different texture atlases every time, which is a handful - some of these could potentially be merged to reduce the number of atlases the game needs to keep track of.

Merging mob_effects.png-atlas into gui.png-atlas is an obvious first step - the mob effects atlas only exists because the split of mob effect icons happened in 1.14 whereas splitting every other GUI element happened in 1.20.2 - if they had happened at the same time, mob effects undoubtedly would have been part of the GUI atlas since they're GUI sprites. This may also fix MC-277486.

As for other atlases, it may be a good idea to merge at least some of the existing block entity atlases. The chests and signs atlases are small enough on their own that they could possibly be merged into a single combined atlas. Beds and shulker boxes could also be merged into this, since there are only 16/17 color variants for each. The armor trims atlas is huge and definitely should not be touched.