test-gles2-context: Read pixels using GL_RGBA not GL_RGB
test-gles2-context: Read pixels using GL_RGBA not GL_RGB

The GLES2 spec only guarantees calling glReadPixels with GL_RGBA and
an implementation specific format. Mesa seems to now reject reading
with GL_RGB so the test had started failing. This fixes it to just
read using GL_RGBA.

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

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