increase morph targets to 256

[robtfm]

Objective

our current morph target / blend shape limit is 64 per model, documented as "to support all hardware". many assets use more than this ("over a hundred" is common), so i'd like to increase this limit.

the relevant hardware constraint is uniform buffer size (usage in morph.wgsl). morph targets use 4 bytes per entry, and wgpu's downlevel uniform buffer size limit is 16k so we have a lot of room from the 256 bytes we are currently limiting to, while still supporting the vast majority of hardware.

Solution

increase the limit to 256 targets / 1k.

we also increase the input to the buffer to this 1k size: this was not done previously but worked as long as the min buffer alignment was 256 (which it probably always is at the moment) as it was sized up to match alignment previously.

there may be a slight performance impact from using a larger size in all cases but it should be negligible.

[alice-i-cecile]

This is very controversial: see the discussion in #21256 and the objections from @james7132.

I agree that we need to allow users to lift this, but y'all need to come to a consensus about the best approach forward.

[robtfm]

it looks like the controversial aspect was exceeding 64kb uniform buffer size, i don't believe we'd do that until we went past 16384 max morph targets (@james7132).

apologies if i'm missing something.