The varying validation is being refactored to better handle separable
programs. Some additional validation is being added as well regarding:
- Number of varyings (inputs vs outputs)
- Precision

This refactor will also allow ProgramPipelines to validate varyings
between shaders that are in different Programs.

Bug: angleproject:3570
Change-Id: I13f324da2ffea039e696962d6971a54f7a7b6f77
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2083847
Commit-Queue: Tim Van Patten <timvp@google.com>
Reviewed-by: Courtney Goeltzenleuchter <courtneygo@google.com>