This updates the code in several places to support Geometry and
Tessellation Evaluation shaders to output transform feedback.

Does not turn on any new tests but enables support for XFB when we
turn on GS/TS.

Bug: angleproject:3571
Bug: angleproject:3572
Change-Id: I6dcb768f2df4eeee81a4a500e999fcf16716f58f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2581941Reviewed-by: Tim Van Patten <timvp@google.com>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>