As part of our on-going goal to remove our dependence on a global Cogl
context this patch adds a pointer to the context to each CoglTexture
so that the various texture apis no longer need to use
_COGL_GET_CONTEXT.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 83131072eea395f18ab0525ea2446f443a6033b1)