Implementing a WebGL spec change:
https://github.com/KhronosGroup/WebGL/pull/2658
Fixes WebGL 2 conformance test:
conformance2/transform_feedback/same-buffer-two-binding-points.html

Also applying the change to D3D backends because they can't support
multiple transform feedback outputs in one buffer.

Bug: chromium:866089
Change-Id: I8d7eda14225c13efb7ca1ed974239332be4e79a6
Reviewed-on: https://chromium-review.googlesource.com/c/1347749
Commit-Queue: James Darpinian <jdarpinian@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>