Use ImageIndex in getRenderTarget. Also change ImageIndex to allow invalid indexes for layer. In 3D indexes, sometimes the layer parameter is invalid, indicating we index an entire mip level instead of a layer of a mip level. BUG=angle:741 Change-Id: I7b410f9aaf568f215bb93ae063934669a8fa09bd Reviewed-on: https://chromium-review.googlesource.com/218312 Reviewed-by: Geoff Lang <geofflang@chromium.org> Tested-by: Jamie Madill <jmadill@chromium.org>