This adds a conformance test that creates a GLES2 context via the cogl
api and verifies clearing an offscreen framebuffer via the gles2 api,
and switching back and forth between the Cogl and GLES2 apis.
Reviewed-by: Neil Roberts <neil@linux.intel.com>
(cherry picked from commit 9369c60a596c0cbc7a8bb9a45d7b8ffb6a848311)
