Add identifier to mark drawables

[fenjalien]

A new key should be added to drawables that come from marks so they can be identified by other functions so they can be handled differently. This would be useful for such cases as intersections (don't intersect marks), merge-path (don't join marks) and decorations (don't decorate the marks). I'm not sure about the behaviour of a bounding box however.

The name could be as verbose as is-mark or as a more general approach ignore in the event other elements would produce similar outputs.

If ignore is chosen its difference to hide should be noted, ignored drawables still get drawn to the canvas but won't necessarily get acted upon.