This PR is based on top of #12982
# Objective
- Mesh2d currently only has an alpha blended phase. Most sprites don't
need transparency though.
- For some 2d games it can be useful to have a 2d depth buffer
## Solution
- Add an opaque phase to render Mesh2d that don't need transparency
- This phase currently uses the `SortedRenderPhase` to make it easier to
implement based on the already existing transparent phase. A follow up
PR will switch this to `BinnedRenderPhase`.
- Add a 2d depth buffer
- Use that depth buffer in the transparent phase to make sure that
sprites and transparent mesh2d are displayed correctly
## Testing
I added the mesh2d_transforms example that layers many opaque and
transparent mesh2d to make sure they all get displayed correctly. I also
confirmed it works with sprites by modifying that example locally.
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## Changelog
- Added `AlphaMode2d`
- Added `Opaque2d` render phase
- Camera2d now have a `ViewDepthTexture` component
## Migration Guide
- `ColorMaterial` now contains `AlphaMode2d`. To keep previous
behaviour, use `AlphaMode::BLEND`. If you know your sprite is opaque,
use `AlphaMode::OPAQUE`
## Follow up PRs
- See tracking issue: #13265
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Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
Co-authored-by: Christopher Biscardi <chris@christopherbiscardi.com>
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