[naga] Add packed as a keyword for GLSL

[kjarosh]

Turns out that sometimes packed is a keyword, and the produced GLSL had syntax errors due to that.

Connections
Fixes #5853.

Description
Sometimes packed is a keyword in GLSL, and naga was preserving variables named that way, which caused syntax errors.

Testing
I executed the following command and observed that the packed variable name was suffixed with an underscore. The input file was the original file that caused syntax errors for some users (https://github.com/ruffle-rs/ruffle/blob/036839fb1f382852711e0c1e270fa3e69a3a540a/render/wgpu/shaders/filter/displacement_map.wgsl). Others have confirmed that renaming the identifier to something else than packed fixes those issues.

cargo run --package naga-cli displacement_map.wgsl output.vert --entry-point main_vertex

Checklist

• Run cargo fmt.
• Run cargo clippy (it produces warnings but my patch does not introduce any). If applicable, add:
  • --target wasm32-unknown-unknown
  • --target wasm32-unknown-emscripten
• Run cargo xtask test to run tests.
  • The same set of ~45 tests fail both before and after this PR, so... it's stable at least?
• Add change to CHANGELOG.md. See simple instructions inside file.

[torokati44]

I'm wondering whether there are any other similar keywords that should also be added. Such as the std### ones.

[kjarosh]

I was wondering that too, but I couldn't find any evidence on the internet that any of them should be treated as a keyword. However, we may prefer falsely claiming that something is a keyword to falsely claiming that something isn't.

There's also an option to test those suspected keywords on a platform where packed is a keyword.

[torokati44]

I tried looking at the documents linked in the comments within this list, and search for packed in them, seeing what's next to it in there.

[kjarosh]

@sombraguerrero has confirmed that whilst packed is a keyword on their platform, other similar identifiers (std140,std430,row_major,location,triangles,index,r8) are not.

I did some tests myself (but not using wgpu, as OpenGL does not work for me in wgpu). And packed on my platform:

• is not a keyword in OpenGL,
• is not a keyword in OpenGL ES,
• is a keyword in WebGL 1.0,
• is not a keyword in WebGL 2.0.

I also tried other similar identifiers in WebGL, but it really seems that packed is the only one.

[teoxoy]

Thanks!

[teoxoy]

I looked at the specs and we seem to be missing a few more, I will open a PR for the remaining ones.