[graphite] Add a CombinationBuilder

This also makes the PaintCombinations object opaque to the user.

The eventual goal is to have all the bits of the combination allocated
in the builder's arena.

Bug: skia:12701
Change-Id: I9de8ab1fb6a340cee84c2bd437ddd9b0aa3ed6ea
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/541222
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
4 files changed