Support for grayscale textures

[almarklein]

Currently, textures of format r8unorm and r8uint work, but r16sint, r32sint and r32float fail. I've not yet been able to find out why.

[Korijn]

Are these formats known to work in wgpu-native upstream?

[almarklein]

When I asked last week or so, they responded to not have trouble with them earlier. But then again, it does work with uint8 textures. At the time I did not have the tests to say what exactly worked and what not. Let's wait for their next release and if it then still fails I'll ask for help.

[almarklein]

gfx-rs/wgpu#575

[almarklein]

I tweaked a Rust example to use R16Sint and R32Sint textures and that seemed to work. Should compare that example's wgpu calls, and compare the SpirV that comes off that fragment shader with our SpirV ...