glsl: fix a race condition when inserting new types

glsl: fix a race condition when inserting new types

By splitting glsl_type::mutex into two, we can avoid dropping the hash
mutex while creating the new type instance (e.g. struct/record,
interface).

This fixes a time-of-check/time-of-use race where two threads would
simultaneously attempt to create the same type but end up with different
instances of glsl_type.

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>