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