We would allow the app to attach layers that were out-of-bounds. Fix
this by checking against the underlying resource dimensions. Also
rework the code a bit to clean up the texture size query, which is
available from the ImageDesc.

BUG=angleproject:869

Change-Id: I984f1db16daea6ca650d795884d8ec2cb8f05ebb
Reviewed-on: https://chromium-review.googlesource.com/313991
Tryjob-Request: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>