naga: Add support for GLSL signed/unsigned samplers

[DavidPeicho]

Description

Add support for usampler* and isampler for GLSL input. Example of valid input:

#version 440 core

layout(set = 0, binding = 0) uniform utexture2D tex;
layout(set = 0, binding = 1) uniform sampler samp;

layout(local_size_x = 8, local_size_y = 8) in;
void
main()
{
  vec4 data = vec4(texture(usampler2D(tex, samp), vec2(0.0, 1.0)));
}

Testing

Updated the test samplers.frag.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy.
• Run cargo xtask test to run tests.
• Add change to CHANGELOG.md. See simple instructions inside file.

[teoxoy]

Thanks!