- Built-in variable gl_ClipDistance has been added to compiler.
- Desktop GL: gl_ClipDistance is supported since GL 3.0. Enable/Disable
each gl_ClipDistances[i] works out of the box via glEnable().
- Vulkan/Metal: Use uniform variable to control writing to each
gl_ClipDistance. One bit flag controls one element in the gl_ClipDistance
array. The writing to the disabled element in vertex shader will be
ignored, and turned into zero assignment instead.
- Direct3D/Mobile GL: Not implemented yet.
- Added ClipDistanceTest to gl_tests and compiler unittests.
- GL_APPLE_clip_distance is a subset of GL_EXT_clip_cull_distance, so
GL_EXT_clip_cull_distance could be implemented in future if needed.

Bug: angleproject:4452
Change-Id: I571ac8b56826989808a680226a04bec4cf59988e
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2084324
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>