Implement an IOSurface-backed pBuffer surface for the Vulkan backend
on Mac, through SwANGLE. ANGLE will pass a raw pointer to Swiftshader
and handle locking/unlocking the IOSurface.

Bug: chromium:1015454
Change-Id: Ia3ead55334736003d405b54ba8dcc7701706fbb2
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1965434
Commit-Queue: Jonah Ryan-Davis <jonahr@google.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>