Clean up BlitGL 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>