Vulkan: Implement glTexStorageMem2DEXT 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/+/1552029 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Michael Spang <spang@chromium.org>