This adds a utility function for inferring a CoglPixelFormat from a
set of channel masks, a bits-per-pixel value, a pixel-depth value and
pixel byte order.
This plan is to use this to improve how we map X visuals to Cogl pixel
formats.
This patch was based on some ideas from Damien Leone <dleone@nvidia.com>
https://bugzilla.gnome.org/show_bug.cgi?id=660188
Reviewed-by: Neil Roberts <neil@linux.intel.com>