glsl: Fix prorgram interface query locations biasing for SSO.

glsl: Fix prorgram interface query locations biasing for SSO.

With SSO, the GL_PROGRAM_INPUT and GL_PROGRAM_OUTPUT interfaces refer to
the first and last shader stage linked into a program.  This may not be
the vertex and fragment shader stages.

So, subtracting VERT_ATTRIB_GENERIC0 and FRAG_RESULT_DATA0 is bogus.
We need to subtract VERT_ATTRIB_GENERIC0 for VS inputs,
FRAG_RESULT_DATA0 for FS outputs, and VARYING_SLOT_VAR0 for other cases.

Note that built-in variables get a location of -1.

Fixes 4 dEQP-GLES31.functional.program_interface_query tests:
- program_input.location.separable_fragment.var_explicit_location
- program_input.location.separable_fragment.var_array_explicit_location
- program_output.location.separable_vertex.var_array_explicit_location
- program_output.location.separable_vertex.var_array_explicit_location

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Timothy Arceri <timothy.arceri@collabora.com>