Add support for r/rg/rgba-16 and -16ui in compute shaders.

[mcourteaux]

Please carefully review what I did. I only needed rgba16, which worked for me after this change, but this is the first time I'm working with bgfx compute shaders.

[bkaradzic]

Just add variant that you tested and it works... It's better to lack something, than have it untested (it creates more confusion).

[mcourteaux]

If you're concerned about testing, I can only speak to the success of this in Vulkan. Shaderc did compile it for GLSL, SPIRV and Metal (can't do D3D12/HLSL because I don't have the microsoft shader compiler 🤷🏻).

The source of information was this commit in glslang:

https://github.com/KhronosGroup/glslang/pull/1918/files#diff-2ce711ecd274f146c2ca7969d6de4f729aa9e5350ddde7b7d028c2bb049674de

[mcourteaux]

Curious... If I deliberately put in the wrong format (not matching the actual texture format), it all still just works. As if Vulkan is ignoring the format spec in the shader, and is actually looking at the Texture's pixel format. Is this normal behavior?

[mcourteaux]

Can confirm it also works on MacOS (metal) and Windows (vulkan) by now.