Fix multiview framebuffer completeness check In the process of dropping ANGLE_multiview in favor of OVR_multiview, the framebuffer completeness checks have become stale. In particular: - There is no requirement that the base layer of the attachments match - There _is_ a requirement that base+count layers are within the texture boundaries. Bug: chromium:1264988 Change-Id: I86837b587ad5befaa6a545c5a24507e8dff0b568 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3259272 Reviewed-by: Alexis Hetu <sugoi@google.com> Reviewed-by: Alexis Hétu <sugoi@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>