Adds a CPU readback and conversion path when the destination texture is
not renderable.

BUG=angleproject:1932

Change-Id: I71461ca991dc10dd636ff38e1ae20db2be0f8d63
Reviewed-on: https://chromium-review.googlesource.com/508308
Commit-Queue: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>