Add support for YCbCr_420_888 and YCrCb_420_888 formats,
more commonly known as NV12 and NV21.

Bug: angleproject:5773
Change-Id: I1568d543c221ad110e7f199eb2ffd5df23a6134e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2787251
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>