glsl/linker: outputs in the same location must share interpolation

glsl/linker: outputs in the same location must share interpolation

From ARB_enhanced_layouts:

"[...]when location aliasing, the aliases sharing the location
 must have the same underlying numerical type (floating-point or
 integer) and the same auxiliary storage and
 interpolation qualification.[...]"

Add code to the linker to validate that aliased locations do
have the same interpolation.

Fixes:
KHR-GL45.enhanced_layouts.varying_location_aliasing_with_mixed_interpolation

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