tests: Mark test_framebuffer_get_bits as only working on GL
tests: Mark test_framebuffer_get_bits as only working on GL

It looks like it's not meant to be valid to create a framebuffer with
an alpha-only texture as a render target on GLES. Since the following
Mesa commit, this requirement is now enforced so that the
test_framebuffer_get_bits test fails:

http://cgit.freedesktop.org/mesa/mesa/commit/?id=cf300eaa

Reviewed-by: Robert Bragg <12e9293ec6b30c7fa8a0926af42807e929c1684f@linux.intel.com>

djdeath/cogl-android,djdeath/cogl-android,gcampax/cogl,djdeath/cogl-android,gcampax/cogl,djdeath/cogl,gcampax/cogl,djdeath/cogl,djdeath/cogl,gcampax/cogl