In some cases ANGLE gets an external back buffer texture for our
SwapChain11 class, and in some of these cases we get an offscreen
texture that can't be used as a shader resource. The becomes a problem
for some copy operations that use a shader to convert texture formats.

Work around this problem by making a shadow copy of the texture that
has sampling enabled - it is possible to use CopyResource to copy
between them.

BUG=chromium:752917

Change-Id: Ib757949d3d06295a118b055bf37311f820f7149c
Reviewed-on: https://chromium-review.googlesource.com/638551Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>