nir+mesa/st+r600: Add lower_to_scalar callback to complier opetions and use it in glsl_to_nir

lower_to_scalar makes it possible filter the instructions to be lowered, but in mesa/st a filter defined by the driver could not be passed in, so add the according callback pointer to the compiler options, pass it in mesa/st, and enable lower_to_scalar with the filter callback in r600.