adg-demo: s/shape/body/

Renamed shape to body to match the adg-demo program in adg-lua.
adg-demo: favoring simplicity over rendering quality

The modeling and rendering were more complex in order to avoid rendering
of multiple lines in the same place. Just ignore it and simplify
modeling as much as possible.
adg: allow-none in adg_path_reflect()

The API accepts a NULL axis, with a fallback to y=0.
adg: improved AdgDress annotations
test: added AdgDress testing
adg: cosmetic improvements in adg-dress-builtins.h
adg: check dresses have unique ids