st/mesa: add support for viewport index semantic

This adds GS output and FS input support, even though FS input
support isn't supported until GLSL 4.30 from what I can see.
Signed-off-by: Dave Airlie <airlied@redhat.com>

src/mesa/state_tracker/st_program.c

@@ -572,6 +572,11 @@ st_translate_fragment_program(struct st_context *st,
             input_semantic_index[slot] = 0;
             interpMode[slot] = TGSI_INTERPOLATE_CONSTANT;
             break;
+         case VARYING_SLOT_VIEWPORT:
+            input_semantic_name[slot] = TGSI_SEMANTIC_VIEWPORT_INDEX;
+            input_semantic_index[slot] = 0;
+            interpMode[slot] = TGSI_INTERPOLATE_CONSTANT;
+            break;
          case VARYING_SLOT_CLIP_DIST0:
             input_semantic_name[slot] = TGSI_SEMANTIC_CLIPDIST;
             input_semantic_index[slot] = 0;
@@ -1027,6 +1032,10 @@ st_translate_geometry_program(struct st_context *st,
             gs_output_semantic_name[slot] = TGSI_SEMANTIC_PRIMID;
             gs_output_semantic_index[slot] = 0;
             break;
+         case VARYING_SLOT_VIEWPORT:
+            gs_output_semantic_name[slot] = TGSI_SEMANTIC_VIEWPORT_INDEX;
+            gs_output_semantic_index[slot] = 0;
+            break;
          case VARYING_SLOT_TEX0:
          case VARYING_SLOT_TEX1:
          case VARYING_SLOT_TEX2: