By using a 2D instead of 2D array view for the render target we can
merge the 'storage' and 'draw' level/layer views.

Bug: angleproject:3975
Change-Id: Icf8c0bf6bec4147595c2b728362abd51bdf03f3e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1841947Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>