Previously, only texture copies used a shader that performed
texelFetch().  To support cubemaps, a hack was used to temporarily
create a 2D array view.

With upcoming support for multisample resolve, more shaders will be
using texelFetch() all requiring this workaround.  This change instead
makes sure that a separate view is created for cubemaps for the purpose
of being used with these shaders.  As a result, we have three logical
views on textures and render targets:

- Draw: a view that can be used as a color/depth/stencil attachment
- Read: a view that can be used to sample from
- Fetch: a view that can be used to fetch from

The fetch view is generally the same as the read view, except for cube
maps.

Bug: angleproject:3200
Change-Id: I21547f728c16f0aa8f0fcae152c400b5cc1565da
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1628585
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>