gallium: Handle client-supplied edgeflags.

Also, implement support in the draw module.  We were hardwiring these
to one for quite a long time...

Currently using a draw_set_edgeflags() function, may be better to push
the argument into the draw_arrays() function.  TBD.

src/gallium/include/pipe/p_context.h

@@ -57,6 +57,14 @@ struct pipe_context {
 
    void (*destroy)( struct pipe_context * );
 
+   
+   /* Possible interface for setting edgeflags.  These aren't really
+    * vertex elements, so don't fit there.
+    */
+   void (*set_edgeflags)( struct pipe_context *,
+                          const unsigned *bitfield );
+
+
    /**
     * VBO drawing (return false on fallbacks (temporary??))
     */