accessors for RunBuffer as points or rsxforms

trying to reduce casts at call-sites

Bug: skia:
Change-Id: I7c74cfd89111f7a197cbb1d77b499e2adc193579
Reviewed-on: https://skia-review.googlesource.com/c/180363
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Reed <reed@google.com>
7 files changed