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 <robert@linux.intel.com>
(cherry picked from commit cfb0859cab843b000f4baa3ca155a245730edcfa)