Update ImageHelper to be able to copy both the depth and stencil
aspects of a DS image to a buffer. The aspects are copied separately
with the depth data preceding the stencil data.

This allows dEQP-GLES31.functional.stencil_texturing.misc.base_level
test to pass. Added exception for ANDROID VULKAN where test still
fails and new tracking bug (4080) for this case.

Bug: angleproject:3949
Bug: angleproject:4080
Change-Id: Ib6104d7fa9f516154131f3e82161078ba216cfe1
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1897649Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Tobin Ehlis <tobine@google.com>