Stop tying the viewport and the scissor together. Instead, we simply
use a very large (0->maxInt) scissor when scissor isn't enabled
and we use the clipped scissor to the renderArea size when its
enabled.

Bug: angleproject:2443

Change-Id: If7454793a050b1833c7d3166ea6b380192085c8f
Reviewed-on: https://chromium-review.googlesource.com/1006996
Commit-Queue: Luc Ferron <lucferron@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>