Forbidden areas

[perliedman]

It should be possible to add forbidden areas as objects.

Drawing them mostly work as white-out areas, but styling needs to be different. We can probably use the same technique as we do in SVG (canvas seem to support SVG fills), but we need to make sure scaling is done correctly.

Also, we need to add the same for map print.

@wlfrdssn seems to have done some work in this area but was unable to complete it.

(An alternative, that is currently hard to implement, is to add forbidden areas as new objects to the OCAD map file in memory, and let the OCAD drawing logic handle forbidden areas. This could give a lot of potentially interesting functionality down the road, as draw white out areas between OCAD layers. It's not really possible to do without some major work in ocad2geojson, like being able to construct symbols and objects without an existing map file.)