mesa: fix another "out of samplers" problem

mesa: fix another "out of samplers" problem

Now only the samplers that are actually used by texture() functions are
saved in the uniform variable list.  Before, we could run out of samplers
if too many were declared while only some of them were actually used.

(cherry picked from commit 1fad6ccb756ae33ca3115f59c99ca8abbeb0321e)

Conflicts:

	src/mesa/shader/slang/slang_link.c