nir/linker: take into account hidden uniforms

nir/linker: take into account hidden uniforms

So they are not exposed through the introspection API.

It is worth to note that the number of hidden uniforms of GLSL linking
vs SPIR-V linking would be somewhat different due the differen order
of the nir lowerings/optimizations.

For example: gl_FbWposYTransform. This is introduced as part of
nir_lower_wpos_ytransform. On GLSL that is executed after the IR-based
linking. So that means that on GLSL the UniformStorage will not
include this uniform. With the SPIR-V linking, that uniform is already
present, but marked as hidden. So it will be included on the
UniformStorage, but as hidden.

One alternative would create a special how_declared for that case, but
seemed an overkill. Using hidden should be ok as far as it is used
properly.

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