WebGL CopyTexImage needs to zero the part of the texture corresponding
to area outside the framebuffer, so we zero the whole texture then clip
the read area.
The clipping also avoids problems with code lower down that isn't prepared
for read areas not entirely within the framebuffer.

Enable corresponding test.

BUG=angleproject:1815

Change-Id: Ia7e0243ca72fa7c8f5bacda4d2022061d6a6d4f0
Reviewed-on: https://chromium-review.googlesource.com/551056Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Frank Henigman <fjhenigman@chromium.org>