This removes several uses of _COGL_GET_CONTEXT in cogl-atlas-texture.c.
Notably this involved making CoglPangoGlyphCache track an associated
CoglContext pointer which cogl-pango can pass to
_cogl_atlas_texture_new_with_size().
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit d66afbd0758539330490945c699a05c0749c76aa)
fa5a9c88fe