ANGLE was hitting an issue running on llvmpipe because llvmpipe doesn't
properly implement texture formats with multisampling, but claims
otherwise to be OpenGL 3.3 compliant. ANGLE did not validate the claim
on the backend, so the frontend was hitting an ASSERT that failed.
This patch makes sure to validate this part of the spec, and limits the
driver to OpenGL 2 if it's not conformant.

Bug: 976382
Change-Id: I6ad6d757e26f90068df83aeb3caf7685aa2f1c07
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1853889
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>