From: Carl Worth <cworth@cworth.org>

Don't copy unused bits to an alpha channel

This bug was identified by cairo's rgb24-ignore-alpha test which
blends RGB24 over ARGB32 and notices that "alpha" values are
making it from the source to the destination.

Index: a/pixman/pixman-pict.c
===================================================================
--- a/pixman/pixman-pict.c b8b7b767b3ed1fc57ba177c38187e1eb3daf4de8
+++ b/pixman/pixman-pict.c c7d73fc532c2da313041906ac0f48dba1b10d3bc
@@ -1778,7 +1778,6 @@ pixman_image_composite (pixman_op_t     
 		    break;
 		case PIXMAN_x8r8g8b8:
 		    switch (pDst->bits.format) {
-		    case PIXMAN_a8r8g8b8:
 		    case PIXMAN_x8r8g8b8:
 #ifdef USE_MMX
 			if (pixman_have_mmx())
@@ -1790,7 +1789,6 @@ pixman_image_composite (pixman_op_t     
 		    }
 		case PIXMAN_x8b8g8r8:
 		    switch (pDst->bits.format) {
-		    case PIXMAN_a8b8g8r8:
 		    case PIXMAN_x8b8g8r8:
 #ifdef USE_MMX
 			if (pixman_have_mmx())