[graphite] Add round cap support to CircularArcRenderStep.

Round caps are supported by testing the distance from the centers of
the caps to the width of the stroke, and unioning the result with the
existing coverage.

Bug: b/366426193
Change-Id: I81b8fccde6aadc1817ca3eb226661c1d0d549a32
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/914357
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed