GL: Implement clip distance state emulation Pass the current set of enabled clip distances to vertex shaders via an internal uniform and dynamically set disabled elements to zero. Bug: angleproject:7880 Change-Id: I709d31dc7ca0606decf49adf674460a941837683 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/4094314 Commit-Queue: Alexey Knyazev <lexa.knyazev@gmail.com> Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>