…es (#6697)

# Objective

- Right now we can't really benefit from [early depth
testing](https://www.khronos.org/opengl/wiki/Early_Fragment_Test) in our
PBR shader because it includes codepaths with `discard`, even for
situations where they are not necessary.

## Solution

- This PR introduces a new `MeshPipelineKey` and shader def,
`MAY_DISCARD`;
- All possible material/mesh options that that may result in `discard`s
being needed must set `MAY_DISCARD` ahead of time:
- Right now, this is only `AlphaMode::Mask(f32)`, but in the future
might include other options/effects; (e.g. one effect I'm personally
interested in is bayer dither pseudo-transparency for LOD transitions of
opaque meshes)
- Shader codepaths that can `discard` are guarded by an `#ifdef
MAY_DISCARD` preprocessor directive:
  - Right now, this is just one branch in `alpha_discard()`;
- If `MAY_DISCARD` is _not_ set, the `@early_depth_test` attribute is
added to the PBR fragment shader. This is a not yet documented, possibly
non-standard WGSL extension I found browsing Naga's source code. [I
opened a PR to document it
there](gfx-rs/naga#2132). My understanding is
that for backends where this attribute is supported, it will force an
explicit opt-in to early depth test. (e.g. via
`layout(early_fragment_tests) in;` in GLSL)

## Caveats

- I included `@early_depth_test` for the sake of us being explicit, and
avoiding the need for the driver to be “smart” about enabling this
feature. That way, if we make a mistake and include a `discard`
unguarded by `MAY_DISCARD`, it will either produce errors or noticeable
visual artifacts so that we'll catch early, instead of causing a
performance regression.
- I'm not sure explicit early depth test is supported on the naga Metal
backend, which is what I'm currently using, so I can't really test the
explicit early depth test enable, I would like others with Vulkan/GL
hardware to test it if possible;
- I would like some guidance on how to measure/verify the performance
benefits of this;
- If I understand it correctly, this, or _something like this_ is needed
to fully reap the performance gains enabled by #6284;
- This will _most definitely_ conflict with #6284 and #6644. I can fix
the conflicts as needed, depending on whether/the order they end up
being merging in.

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## Changelog

### Changed

- Early depth tests are now enabled whenever possible for meshes using
`StandardMaterial`, reducing the number of fragments evaluated for
scenes with lots of occlusions.