We can use image index iterators to iterate over all images in a
Texture. This allows us to do some operations in TextureD3D rather
than in the typed subclasses and save on some repeated code.

BUG=angle:729

Change-Id: I3ba47b2eebad2cfca313117fd501ff76d5107044
Reviewed-on: https://chromium-review.googlesource.com/219834Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Brandon Jones <bajones@chromium.org>