Increase GL MAX_PUSH_CONSTANTS from 16 to 64

[Dinnerbone]

Checklist

• Run cargo clippy.
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
None.

Description
I've found that using the emulated push constants is significantly faster than buffer juggling on our own on the webgl backend. However, it's limited to 16*4 bytes right now, which is too small for our needs.

I can't find any rationale behind it behind just 16*4, it's possible it was a naga limitation but it doesn't seem to be the case anymore? The docs for Limits::max_push_constant_size suggest it should be 256 on GLES so this increases it to that, which can hold a couple of matrices nicely.

If I'm wrong and there's a limitation I missed somewhere and this doesn't actually work somehow, then okay, nevermind! But I've tested this out on Ruffle and it seems to work perfect and performant for us.

Testing
Manually tested and it works fine, not sure it deserves its own unit test.

[codecov-commenter]

Codecov Report

Merging #3374 (6738aff) into master (48fbb92) will decrease coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3374      +/-   ##
==========================================
- Coverage   64.70%   64.63%   -0.07%     
==========================================
  Files          66       86      +20     
  Lines       37336    42722    +5386     
==========================================
+ Hits        24159    27615    +3456     
- Misses      13177    15107    +1930     

Impacted Files	Coverage Δ
wgpu-hal/src/gles/mod.rs	61.53% <ø> (ø)
wgpu-hal/src/gles/device.rs	81.40% <100.00%> (+1.32%)	⬆️
wgpu-core/src/hub.rs	60.98% <0.00%> (-3.60%)	⬇️
wgpu-core/src/instance.rs	65.67% <0.00%> (-2.04%)	⬇️
wgpu/src/lib.rs	51.06% <0.00%> (-0.28%)	⬇️
wgpu-hal/src/auxil/dxgi/exception.rs	87.30% <0.00%> (ø)
wgpu-hal/src/dx12/device.rs	88.45% <0.00%> (ø)
wgpu-hal/src/dx12/command.rs	73.46% <0.00%> (ø)
wgpu-hal/src/dx12/adapter.rs	81.38% <0.00%> (ø)
wgpu-hal/src/dx12/conv.rs	84.22% <0.00%> (ø)
... and 27 more

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[cwfitzgerald]

LGTM, 256 is the standard push constant limit anyway

[cwfitzgerald]

You have a test failing - but also we might have issues with the number of uniforms - we probably should at least be consulting the limits set by GL

[Dinnerbone]

Darn, I couldn't test that here trivially. It looks like the test was set up assuming a size of 16 though, if I'm reading this right. Will need to find a way to adjust that.

[Dinnerbone]

Actually, am I right in reading that this test wouldn't run if the push constants limit was less than 128? If so, this test was implicitly skipped until now and has never run for GLES. I can still find a way to make it work though!

wgpu/wgpu/tests/shader/struct_layout.rs

Lines 221 to 224 in f0f700c

	.limits(Limits {
	max_push_constant_size: MAX_BUFFER_SIZE as u32,
	..Limits::downlevel_defaults()
	}),

[cwfitzgerald]

Heh, you are. If you can make it work great! Otherwise you can mark it as a failure and file a followup issue.

[Dinnerbone]

Looked into this a bit, I'll mark it as failing and open an issue. In short:
The test makes a PushConstantRange(0..128) whilst only allocating (for example) vec2<f32> in the shader. When the render pass tries to use the push constants, in GL that means uploading uniforms and since there's only actually a few bytes for uniforms... that's an error.

It definitely didn't work before this PR, but it was ignored until now. I'm not sure how this is supposed to work though, how do other backends handle var<push_constant> input: CustomStruct; where struct CustomStruct { member: vec2<f32>, } whilst also allowing the user to say "I have 128 bytes", and not error out?

[cwfitzgerald]

where struct CustomStruct { member: vec2, } whilst also allowing the user to say "I have 128 bytes", and not error out?

The shader is just the view into the push constants - you're allowed to push up more and only view part of it.