The incomplete texture handling is similar between the D3D and Vulkan
back-ends. We create 1x1 textures, initialize them to black, and bind
them when we detect incomplete textures. We would also bind incomplete
textures when we detect feedback loops. In the GL back-end, we
wouldn't detect feedback loops, and would allow the driver to handle
incompleteness.

Instead implement this in a shared helper class, and do the feedback
loop detection in the front-end for every back-end. This makes our
behaviour more consistent between back-ends, and prevents undefined
behaviour.

Because initializing multisample textures is tricky (they
can't be updated with TexImage calls) we do a bit of a workaround so
the back-end can clear the incomplete multisample texture initially.

This progresses the initial Vulkan textures implementation.

BUG=angleproject:2167

Change-Id: I79ddcc0711fcc986f2578a52ac6f701231d241ac
Reviewed-on: https://chromium-review.googlesource.com/700993Reviewed-by: Yuly Novikov <ynovikov@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>