Instead always use 2D array views when rendering to cube maps. We need
to keep the "fetch" image view for the main image for emulating
seamless cube map textures on GL 2.0. We also use it for some texture
copying init. These uses could potentially be replaced with sample
instructions if we wanted to remove this last "fetch" view.

Bug: angleproject:3975
Change-Id: I90f9be9fc6542d8ec27f6722132c1b27acf72176
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1842226Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>
Reviewed-by: Cody Northrop <cnorthrop@google.com>
Commit-Queue: Jamie Madill <jmadill@chromium.org>