Support for COLR foreground color in CoreText.

COLR fonts can use a magic palette index value 0xFFFF to indicate the
foreground color of the drawing context. Modify the CoreText port to
properly indicate when the foreground color is needed and use it.

Bug: skia:12576
Change-Id: I14818b34fbe40ee508ed720510be53bc289c748a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/509178
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
4 files changed