gallium: Introduce flatshade_first field to rasterizer_state.

This bit tells us which vertex of the primitive is used to
propagate color for the remaining vertices if flatshade mode.
diff --git a/src/gallium/include/pipe/p_state.h b/src/gallium/include/pipe/p_state.h
index 3e531c4..a2bd8c6 100644
--- a/src/gallium/include/pipe/p_state.h
+++ b/src/gallium/include/pipe/p_state.h
@@ -112,6 +112,7 @@
    unsigned bypass_clipping:1;
    unsigned bypass_vs:1; /**< vertices are already fully transformed */
    unsigned origin_lower_left:1;  /**< Is (0,0) the lower-left corner? */
+   unsigned flatshade_first:1;   /**< take color attribute from the first vertex of a primitive */
 
    float line_width;
    float point_size;           /**< used when no per-vertex size */