This is a simple virtual method which gives an ImageIndex at a
particular level and layer. It's useful for handling all the texture
types uniformly, which all treat levels and layers differently.

BUG=angle:729

Change-Id: I72db560287272c195b1ff8555f99ab2258ac7e6d
Reviewed-on: https://chromium-review.googlesource.com/219835Tested-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>