glsl/linker: validate explicit locations for SSO programs

glsl/linker: validate explicit locations for SSO programs

v2:
- we only need to validate inputs to the first stage and outputs
  from the last stage, everything else has already been validated
  during cross_validate_outputs_to_inputs (Timothy).
- Use MAX_VARYING instead of MAX_VARYINGS_INCL_PATCH (Illia)

Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Ilia Mirkin <imirkin@alum.mit.edu>