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/218312Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>