Set SkPath.fLastMoveToIndex field from SkPathBuilder

This is a "legacy" field in SkPath, and only needed for editing the
path (in funny cases, such as a relative verb or missing moveto).
When we finally make SkPath immutable, we won't need this field at all.

Note: this CL "fixes" the last 2 columns in path_append_extend gm.
They should appear the same as the previous 2 columns.

Change-Id: Ia5f2e8ec586b5f5189fc3ac2cd513fe89d31cd22
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/436958
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: William Hesse <whesse@google.com>
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Mike Reed <reed@google.com>
4 files changed