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/+/2676201Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Alexis Hétu <sugoi@chromium.org>