lima/nir: Add gl_FragCoord handling
This adds the functionality to deal with the fragment shader input variable "gl_FragCoord". lima
v5:
- move nir lower pass to core nir
- style: put var declaration to first assignment
v4:
- reorder if
v3:
- move builder into function loop
- add func->impl check
- remove some comment
- split into nir and ppir patch
v2:
- move lowering to nir
- rebase on master