textureLoad and textureStore need bounds checks

[jimblandy]

For several of our back ends, the implementations of WGSL's textureStore and textureLoad exhibit undefined behavior if coordinates, array indices, sample indices in multisampled texels, or mip levels are out of bounds. This means that, to prevent undefined behavior on the web, Naga must be able to inject bounds checks into the generated code when used by WebGPU.

This can probably use the same IndexBoundsCheckPolicy enum as used for array/vector/matrix accesses now. However, Naga should let clients control the image bounds check policy separately: some devices can check image bounds themselves, presumably more efficiently than Naga can, and in those cases we should leave it to the driver.

Back ends needing work (add specific issues / PRs as filed):

• glsl
• hlsl
• msl
• spv
• wgsl

[jimblandy]

Note incoming changes to WGSL about how this should behave:
https://github.com/gpuweb/gpuweb/pull/1906/files

[jimblandy]

MSL implemented in #1730.
HLSL bounds checks are handled by the platform.

[jimblandy]

I have no plans to implement this for WGSL or GLSL. Implementing bounds checks for a back end takes me quite a while, and I want to focus most of my attention on what Firefox needs.

[jimblandy]

@cwfitzgerald points out that we can say that WGSL is covered, because it effectively passes through the bounds checks implemented by whatever back end it targets.

[jimblandy]

@JCapucho has expressed a willingness to work on the GLSL back end, so let's leave this bug open until that's done.