Extract a subclass for binning stroke instances

This will make it easier to add "dynamic" attribs like stroke width or
color.

Bug: chromium:1172543
Bug: skia:10419
Change-Id: Ia8326c80fb2fe3ab630b03e9bfdc6368385176b0
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/368956
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Greg Daniel <egdaniel@google.com>
3 files changed