Until this patch multi-view layered framebuffers used to be cleared by
attaching a single layer of each attachment to a framebuffer and calling
the Clear* command for that internal framebuffer.
According to the GL 4.1+ specifications, Clear* commands clear all of
the layers of an attached 2D texture array. If all of the layers are
active for a multi-view layered framebuffer, then we can directly call
the corresponding Clear* command instead of iterating over each layer
and clearing it.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Ie4dfd9fff47715b502f358272bfc47c0373c4e91
Reviewed-on: https://chromium-review.googlesource.com/649209
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>