The patch modifies the ANGLE_multiview specification so that issuing a
Draw* command has undefined results for side-by-side FBOs if the scissor
test is disabled. Also, clearing a side-by-side framebuffer will result
in clearing the whole content of the specified buffers if the scissor
test is disabled.
StateManagerGL and FramebufferGL are modified to address this change in
the spec.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I39a87d297944f12769dee2ead17b508ac22053db
Reviewed-on: https://chromium-review.googlesource.com/612283
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>