compiler: add a new explicit interpolation mode

This introduces one more interpolation mode INTERP_MODE_EXPLICIT,
which is needed for AMD_shader_explicit_vertex_parameter.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Reviewed-by: Bas Nieuwenhuizen <bas@basnieuwenhuizen.nl>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3578>

src/compiler/shader_enums.h

@@ -667,6 +667,7 @@ enum glsl_interp_mode
    INTERP_MODE_SMOOTH,
    INTERP_MODE_FLAT,
    INTERP_MODE_NOPERSPECTIVE,
+   INTERP_MODE_EXPLICIT,
    INTERP_MODE_COUNT /**< Number of interpolation qualifiers */
 };