The patch fixes a bug with the viewport and scissor state not being
correctly updated for side-by-side framebuffers.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I5f85b8146174d78a363316a66e2761e37305cffe
Reviewed-on: https://chromium-review.googlesource.com/663260
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>