gl: use correct pixman accessor for image data
gl: use correct pixman accessor for image data

Commit 774e5bd36f4 changed
-  dest->source.pixmap.x_image->data +
+  (uint8_t *)pixman_image_get_stride(dest->source.pixmap.pixman_image) +

The correct accessor to use is pixman_image_get_data. Thanks to gcc
4.6 for warning about a "different size" int to pointer conversion.

dezelin/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,Fantu/spice,ya-mouse/spice,alon/spice-server,dezelin/spice,Fantu/spice,Fantu/spice,mathslinux/spice,alon/spice-server,Fantu/spice,dezelin/spice,SPICE/spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,freedesktop-unofficial-mirror/spice__spice,mathslinux/spice,dezelin/spice,fgouget/spice,elmarco/spice,mathslinux/spice,mathslinux/spice,elmarco/spice,ya-mouse/spice,alon/spice-server,ya-mouse/spice,elmarco/spice,alon/spice-server,SPICE/spice,fgouget/spice,SPICE/spice,elmarco/spice,mathslinux/spice,freedesktop-unofficial-mirror/spice__spice,fgouget/spice,dezelin/spice,elmarco/spice,alon/spice-server