For undefined behavior when point rendering without writing to gl_PointSize.
Even the comment used to say "This is stictly speaking not an error".

BUG=angleproject:1660

Change-Id: Ic8b33689151c9812592dd6b22c83a3355b7d6756
Reviewed-on: https://chromium-review.googlesource.com/437746Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>