glsl/linker: assign explicit uniform locations

glsl/linker: assign explicit uniform locations

Patch refactors the existing uniform processing so explicit locations
are taken in to account during variable processing. These locations
are temporarily stored in gl_uniform_storage before actual locations
are set.

UNMAPPED_UNIFORM_LOC marks unset location so that we can use 0 as a
valid explicit location.

When locations are set, UniformRemapTable is first populated with
uniforms that have explicit location set (inactive and active ones),
rest are put after explicit location slots.

v2: introduce define for locations that have not been set yet (Ian)

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>