glsl: reject samplers not declared as uniform/function params earlier

glsl: reject samplers not declared as uniform/function params earlier

This improves consistency with image variables and atomic
counters which are already rejected the same way.

Note that opaque variables can't be treated as l-values, which
means only the 'in' function parameter is allowed.

v2: rewrite commit message

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com> (v1)
Reviewed-by: Marek Olšák <marek.olsak@amd.com> (v2)