[naga: glsl-out] partially fix sampling depth texture in GL backend

[newDINO]

Connections

This partially fix #4524.

Description

Motivatoin

Allow user to sample depth texture when using GL backend.

Main changes

• When writing image types, if the image is a depth image, check if the image is sampled by a comparing sampler, if not, change image type to sampler2D. In order to be able to get the image's corresponding sampler, FunctionInfo::sampler_set is renamed to sampler_map and use HashMap to get sampler.
• Because when sampling a texture using texture() in glsl always returns vec4, add .r to convert it to float when sampling a depth texture.
• Add an additional test. (naga/tests/in/wgsl/4524-sample-depth.wgsl)

Other changes

Running the test changes some of the files in naga/tests/out.

Drawbacks

User still can only use one sampler for each texture in GL backend.
So, if user sample from a depth texture, they can't use textureSampleCompare() on it, vice versa.

Testing

I have run all naga test, but not cargo xtask test, because it doesn't pass before the changes.
I also tested this version of wgpu on my own project.

LLM Use?

Code is written by human.
I used LLM to learn something about OpenGL.

Checklist

• I self-reviewed and fully understand this PR.
• CHANGELOG.md entries for the user-facing effects of this change are present.
• The PR is minimal, and doesn't make sense to land as multiple PRs.
• Commits are logically scoped and individually reviewable.
• The PR description has enough context to understand the motivation and solution implemented.
• (If applicable) WebGPU implementations built with wgpu may be affected behaviorally.
• (If applicable) Validation and feature gates are in place to confine behavioral changes.
• (If applicable) Tests demonstrate the validation and altered logic works.

[newDINO]

There is a similar PR in the past #8696 that is closed.

The solution from the user side is to use texture_2d<f32> in wgsl, and create BindGroupLayoutEntry with TextureSampleType::Float. This way, I can bind depth texture as texture_2d.