Allow reads from a multiview framebuffer with one view It's safe to read from a multiview framebuffer if it is layered and has just one view. The native OVR_multiview spec supports this as well. BUG=angleproject:2062 TEST=angle_end2end_tests Change-Id: I04e1364390574075f7e06e39a64e3bf05a539a05 Reviewed-on: https://chromium-review.googlesource.com/1156509 Commit-Queue: Olli Etuaho <oetuaho@nvidia.com> Reviewed-by: Jamie Madill <jmadill@chromium.org>