Simplify handling of reversing the blitted area and calculate shader
parameters in a way that doesn't require lengthy explanation.

BUG=chromium:830046
TEST=angle_end2end_tests

Change-Id: Ie9bce812be4ef04a969153fc7c484039fc48a198
Reviewed-on: https://chromium-review.googlesource.com/1030172
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>
Reviewed-by: Jamie Madill <jmadill@chromium.org>