Multiview tests now use common helpers to create textures for
multiview framebuffers and often also to attach the textures to the
framebuffers.

The tests now rely on uploaded texture data to initialize the buffers
instead of clearing the framebuffers with glClear.

BUG=angleproject:2765
TEST=angle_end2end_tests

Change-Id: I7d6d63add5943cab610ab888045d5b0f8ba29215
Reviewed-on: https://chromium-review.googlesource.com/1184712Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>