cogl: Learn about gles3

GLES3 adds several features that we already have code paths for, but we're not checking for their existence in GLES. Note that llvmpipe implements GLES3.0 so we do in fact cover this in the self-tests. But conversely we're no longer testing the GLES2 path, so we may want to add another test pass for that with MESA_GLES_VERSION_OVERRIDE=2.0 set.