restructure EdgeClipper to accommodate other callers (e.g. half-plane)

This allows us to clip a path against a half-plane (an arbitrary
diagonal line), in an effort to handle perspective correctly with
the near-plane.

Change-Id: I2082a750acf858340e4500ad46cf581472cefc6b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/253996
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
6 files changed