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_atom_shader.c

@@ -89,6 +89,10 @@ update_fp( struct st_context *st )
    key.clamp_color = st->clamp_frag_color_in_shader &&
                      st->ctx->Color._ClampFragmentColor;
 
+   /* Ignore sample qualifier while computing this flag. */
+   key.persample_shading =
+      _mesa_get_min_invocations_per_fragment(st->ctx, &stfp->Base, true) > 1;
+
    st->fp_variant = st_get_fp_variant(st, stfp, &key);
 
    st_reference_fragprog(st, &st->fp, stfp);
@@ -108,7 +112,7 @@ update_fp( struct st_context *st )
 const struct st_tracked_state st_update_fp = {
    "st_update_fp",					/* name */
    {							/* dirty */
-      _NEW_BUFFERS,					/* mesa */
+      _NEW_BUFFERS | _NEW_MULTISAMPLE,			/* mesa */
       ST_NEW_FRAGMENT_PROGRAM                           /* st */
    },
    update_fp  					/* update */