Flatten (Cmd+E, Alt+Shift+F) and Join selection (Cmd+J) built the result
from each layer's fill outline, so open paths, which have none, were
dropped: two pen paths flattened to "nothing to flatten", and next to a
closed shape they vanished. Vectors now bring their own network (open
paths, unfilled loops and each region's winding stay), mapped into the
parent's space; other layers still flatten to their outline.
Closes#377
AganzeFelicite
e8b181de39