Change snap target of areas from fills to outlines.

[Bonkles]

Currently, the renderer for polygons draws both an outline and an area fill. The fill depends on whether we are using wireframe mode, partial fill mode, or full-fill mode:

area_fills.mov

As of right now- only the fills are targetable for node snapping. This needs to be changed- we should allow only the outlines to be snapped/connected to, but that's not ready yet.

Here's an example of what this looks like when trying to add a new node to a polygon:

node_dropping_onto_fill.mov

The behavior here is not intuitive- the node is being snapped to the edge of the polygon since RapiD thinks the way is being 'snapped' to.

[bhousel]

I've added a bunch of workarounds in the code that should mostly prevent snapping if the click too far away from the shape's edge. I'll leave the issue open, hoping to add a less hacky fix later.