[graphite] Break PaintParams out into its own header

Bug: skia:12701
Change-Id: I8138633dee9eaf65077da8534600393196c7942e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/492404
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
7 files changed