Move canvas helper structs to header

This is necessary cleanup before changing the type of the matrix and
clip stack. That work has landed and reverted several times, so landing
this piece separately, first.

Change-Id: I147e4cc4260fa5e07a0712503f879da120f8466a
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/435278
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed