The patch modifies Clear11 to add support for clearing the color, depth
and stencil attachments to a side-by-side framebuffer:
- Color attachments are cleared through the command ClearView.
- Depth and stencil attachments are cleared by drawing a quad on top of
each view.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I5753a72222216a48cd954eb1219bc58f968735fc
Reviewed-on: https://chromium-review.googlesource.com/603853
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>