When an image is backed by an external source like an AHB
we need to refcount the buffer appropriately to prevent
deletion of the buffer while in use.

Bug: angleproject:5018
Tests: angle_end2end_tests --gtest_filter=ImageTest.SourceAHBTarget2DEarlyDelete*
Change-Id: Ib6318fc3dc442460d1a2bdd6d75a4458895667bb
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2406615
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>