mesa: add IsSample bitfield to gl_fragment_program

Drivers will need to look at this to decide if they need to do
per-sample fragment shader dispatch.

Signed-off-by: Chris Forbes <chrisf@ijw.co.nz>
Reviewed-by: Francisco Jerez <currojerez@riseup.net>

src/mesa/main/mtypes.h

@@ -2132,6 +2132,12 @@ struct gl_fragment_program
     * uses centroid interpolation, 0 otherwise.  Unused inputs are 0.
     */
    GLbitfield64 IsCentroid;
+
+   /**
+    * Bitfield indicating, for each fragment shader input, 1 if that input
+    * uses sample interpolation, 0 otherwise.  Unused inputs are 0.
+    */
+   GLbitfield64 IsSample;
 };