Enable packing extensions for the Vulkan backend The following extensions are ES2 extensions which become core in ES3: - EXT_unpack_subimage - NV_pack_subimage Since the Vulkan backend already fully supports ES3, it should be ok to enable these for ES2 contexts. This fixes all YUV related viz_unittests failures when using SwANGLE with the passthrough command decoder. Bug: chromium:1060139 Change-Id: I24ffd7a58f576799b93fe91e6be4032ca3175eda Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2676201 Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Alexis Hétu <sugoi@chromium.org>