+ Introduce the GL_OES_point_size_array extension for point size array
support.

BUG=angleproject:2306

Change-Id: Ib1a60b7dcd0497eb807f0d3c80bc95b4748d9a96
Reviewed-on: https://chromium-review.googlesource.com/1014282
Commit-Queue: Lingfeng Yang <lfy@google.com>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>