mesa/st: add per sample shading state to fp key and set interpolation

This enables a gallium driver not to care about the semantics of
ARB_sample_shading vs ARB_gpu_shader5 sample attributes. When
ARB_sample_shading-style sample shading is enabled, all of the fp inputs
are marked for interpolation at the sample location.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Reviewed-by: Roland Scheidegger <sroland@vmware.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>

src/mesa/state_tracker/st_program.c

@@ -548,6 +548,9 @@ st_translate_fragment_program(struct st_context *st,
          else
             interpLocation[slot] = TGSI_INTERPOLATE_LOC_CENTER;
 
+         if (key->persample_shading)
+            interpLocation[slot] = TGSI_INTERPOLATE_LOC_SAMPLE;
+
          switch (attr) {
          case VARYING_SLOT_POS:
             input_semantic_name[slot] = TGSI_SEMANTIC_POSITION;