If we make this per-context and create two Cogl contexts, some types
won't re-register, and we'll be in a broken state where some types will
be considered not to be texture types.
https://bugzilla.gnome.org/show_bug.cgi?id=693696
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 567f049d20554bb8ea4e40fa5e72a9fd0bbd409e)