glsl: optimise inputs/outputs with explicit locations

glsl: optimise inputs/outputs with explicit locations

This change allows used defined inputs/outputs with explicit locations
to be removed if they are detected to not be used between shaders
at link time.

To enable this we change the is_unmatched_generic_inout field to be
flagged when we have a user defined varying. Previously
explicit_location was assumed to be set only in builtins however SSO
allows the user to set an explicit location.

We then add a function to match explicit locations between shaders.

V2: call match_explicit_outputs_to_inputs() after
is_unmatched_generic_inout has been initialised.

Cc: Gregory Hainaut <gregory.hainaut@gmail.com>
Reviewed-by: Tapani Pälli <tapani.palli@intel.com>