The patch adds support for clearing the layers of 2D array textures
attached to a multi-view framebuffer. According to the ANGLE_multiview
spec, the layers which are outside of the range
[baseViewIndex; baseViewIndex + numViews) should remain unmodified.
Because the native Clear* commands clear all of the layers, a workaround
is implemented which creates a FBO, attaches a single layer from all
multi-view attachments and clears the contents.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: Ibf711d02046233eed16bdd3f9c96fc38f82ed0a8
Reviewed-on: https://chromium-review.googlesource.com/615242
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>