marker.rs: rename SegmentState states to something more meaningful
marker.rs: rename SegmentState states to something more meaningful

"Start" and "End" don't really say much about the state of processing
the current subpath, which is the whole point of the state machine.
Use NewSubpath and InSubpath instead.

GNOME/librsvg,GNOME/librsvg,GNOME/librsvg,GNOME/librsvg