Textures specified through external images need to
transition to a ReadWrite layout and not ReadOnly.

Bug: angleproject:4791
Change-Id: I8c0d5999d65499de6a36725ed1cc9c390c479c8c
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2270513
Commit-Queue: Mohan Maiya <m.maiya@samsung.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>