glsl/lower_samplers_as_deref: apply bindings for unused samplers
if a sampler is never used (no derefs) then its binding will never be applied here, leaving it with binding=0. this will clobber the real binding=0 sampler in driver backends, leading to errors, so try to iterate using the same criteria as above and apply bindings in the same way
fixes #8974 (closed)
cc: mesa-stable