Was simply a matter of triggering a new write node in the subImage
call.

Bug: angleproject:2495
Change-Id: I06334ba6ee816f671e5c599c8e8f079f56adb25e
Reviewed-on: https://chromium-review.googlesource.com/1028729
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Luc Ferron <lucferron@chromium.org>