This implements support for creating textures that alias vulkan images
allocated inside external memory.

Bug: angleproject:3289

Change-Id: Iad071f353a217793102ae737647c7cd572f7b0ad
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1552029Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Michael Spang <spang@chromium.org>