The D3D11 backend used to calculate the blit areas incorrectly when
the source blit area provided via the API stretched outside the source
framebuffer in all directions.

This is now fixed and the code is also simplified.

BUG=angleproject:2521
TEST=angle_end2end_tests

Change-Id: I46d5de27464a8455b3956c0be598f75550ca1616
Reviewed-on: https://chromium-review.googlesource.com/1032787Reviewed-by: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Olli Etuaho <oetuaho@nvidia.com>