Make GrTriangulator immutable

Marks its methods const and lifts the breadcrumb list out into
function arguments. This is one more step toward the final vision
where GrTriangulator just has an allocator and control knobs, and
everything else is functional.

Bug: skia:10419
Change-Id: I77341c045d481da49ebfee06de5dfc7a2a8a07be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/360956
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Chris Dalton <csmartdalton@google.com>
8 files changed