Similarly to line loops, we fully unpack the index data into a
triangle list.

BUG=angleproject:597

Change-Id: Icf91cb77df0a941640e34a975898543e1dac5752
Reviewed-on: https://chromium-review.googlesource.com/310501Reviewed-by: Geoff Lang <geofflang@chromium.org>
Tested-by: Jamie Madill <jmadill@chromium.org>