SkPath: when creating a new SkPathRef, only copy if old is not empty

This ends up giving a 15% improvement to the micro-benchmark mentioned
in the bug.

Bug: skia:13817
Change-Id: I0304b388a8e88c28153ad3c654139796bb1d19ff
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/591081
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed