endStroke() was a no-op, so addStrokePoint() kept appending every new
pointer-down to the previous stroke (its only segmentation cue was
points.isEmpty()). A second drag after lift got joined into the first,
and toPath() drew them as a single polyline.
Add isFinished flag to HandStroke and implement real endStroke() that
marks the current stroke finished. withAddedPoint() now starts a new
segment when the previous stroke is null, empty, or finished.
xfy
81aeb8e894