Remove GLSL dependency from Gallium by carving out nir_lower_samplers from libglsl
Previously it was only possible to produce NIR that drivers could actually use if TTN (TGSI to NIR) called
gl_nir_lower_samplers_as_deref. This added an undesired dependency on libglsl. This MR fixes that by carving out the non-GL-specific functionality from libglsl into a function named
nir_lower_samplers and removing the call to
gl_nir_lower_samplers_as_deref. TTN now manually counts the textures used instead of relying on that GLSL function.