glamor: Use glamor_composite_with_shader once per group of glyphs.

This shaves CPU time in GL setup.  Performance of rgb24text went from
18400/sec to 23500/sec.

glamor/glamor_render.c

@@ -1113,6 +1113,9 @@ glamor_composite_rects(CARD8 op,
     ValidatePicture(src);
     ValidatePicture(dst);
 
+    if (glamor_composite_with_shader(op, src, NULL, dst, nrect, rects))
+	return;
+
     n = nrect;
     r = rects;