glsl: mark explicit uniforms as explicit in other stages too

glsl: mark explicit uniforms as explicit in other stages too

If shader declares uniform explicit location in one stage but
implicit in another, explicit location should be used. Patch marks
implicit uniforms as explicit if they were explicit in previous stage.
This makes sure that we don't treat them implicit later when assigning
locations.

Fixes following CTS test:
   ES31-CTS.explicit_uniform_location.uniform-loc-implicit-in-some-stages3

v2: move check to cross_validate_globals (Timothy)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>