A branch is added in the geometry shader to select either the
viewport, or texture layer which is being rendered to based on the
value of a uniform in the driver constant buffer. Using this approach
there is no need for separate programs for side-by-side and layered
rendering.

BUG=angleproject:2062
TEST=angle_end2end_tests

Change-Id: I66701164ff02a851c13695d5409f8ad350534e69
Reviewed-on: https://chromium-review.googlesource.com/645547
Commit-Queue: Martin Radev <mradev@nvidia.com>
Reviewed-by: Olli Etuaho <oetuaho@nvidia.com>