cleanup after conversion to RSVGPathData
Now that all shapes build RSVGPathData on their own, parsing path data no longer has to be a special case but can be integrated into rsvg-shapes.c
Also pass around RSVGPathData* instead of cairo_path_t*. This will remove all backend-spacific code from rsvg-shapes.c and remove some code duplication.
Remove argument last_moveto_index from rsvg_path_builder_move_to and rsvg_path_builder_close_path
Store in RSVGPathData if a curveto is given as a shorthand
Correct start comments in rsvg-path.c, move description of RSVGPathData to rsvg-private.h
minor cleanups