marker.rs: Turn Segment from a struct into an enum
marker.rs: Turn Segment from a struct into an enum

This lets us do away with the "is_degenerate: bool" field, and
use Degenerate and LineOrCurve variants of the enum instead.  This
way it is impossible to access fields which are invalid for
the Degenerate variant.

Thanks to Emmanuele Bassi for the suggestion.

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