glsl: inline functions with unsupported return type before converting to nir

glsl_to_nir doesn't expect non-vector/scalar return types in functions.

Fixes: 7e60d5a5
Closes: #3058 (closed)
Closes: #3060 (closed)

I extracted this commit from !5252 (closed) - other one causes regression and requires additional fixes in other code part. This one has a r-b and I'll merge it tomorrow if nothing pops out.