gtk: add support for surface conversion

Also use CAIRO_FORMAT_RGB24 unconditionally.  DisplaySurfaces will never
ever see 8bpp surfaces.  And using CAIRO_FORMAT_RGB16_565 for the 16bpp
case doesn't seem to be a good idea too.

<quote src="/usr/include/cairo/cairo.h">
 * @CAIRO_FORMAT_RGB16_565: This format value is deprecated. It has
 *   never been properly implemented in cairo and should not be used
 *   by applications. (since 1.2)
</quote>

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-id: 1372150134-8590-1-git-send-email-kraxel@redhat.com
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>