Move `data/shaders` to `data/shaders/glsl`
Move `data/hlsl` to `data/shaders/hlsl`
Fix up shader paths in the cpp files to point to the new glsl location.
`data/shaders/hlsl/compile.py` still overwrites the glsl .spv files (for
now).
Issue: #723
The sample count was previously fixed at 8, which is not supported
on some platforms. The code now checks for the maximum supported
sample count and uses that instead.
If you would prefer the upper limit to be fixed at 8, that is
obviously trivial to change.
