[hal] allow for PreMultiplied alpha mode on the web

[uael]

Description
CompositeAlphaMode::PreMultiplied was missing in WebGPU and GLES supported alpha_modes

Testing
Manually for both Android (GLES) and the Web (WebGL and WebGPU)

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[cwfitzgerald]

@grovesNL I'm going away on vacation, could you take a look at this if you have some time?

[cwfitzgerald]

WebGPU is fine, concern about GL

[cwfitzgerald]

I don't think this is right - without telling this to the runtime at all we can't know if these modes are actually supported. I'm not sure how this works with the various platforms, but I don't think we can just change it.

[uael]

It look like premultipliedAlpha is set to true by default https://registry.khronos.org/webgl/specs/latest/1.0/#5.2 and we do not seem to change it

[grovesNL]

It looks like supporting multiple modes could be tricky if we want it to be handled at the WebGL context level, because we'd need to create the context (wgpu's Surface) with opaque vs. premultiplied. There seems like a couple options:

• If we exposed a way to specify whether alpha and premultiplied should be used when creating the initial surface/context, then we could change composite_alpha_modes to return Opaque/PreMultiplied depending on whatever the surface is currently using.
• If we always use premultiplied on the context, I guess we could at least handle both opaque and premultiplied for srgb presentation by handling it in srgb_present.frag. This wouldn't work for non-srgb though where we blit the framebuffer (unless we went through a shader there for the opaque case).