`map.editable` is hot code because it's called frequently by the `isHiddenX`
tests in `features.js`. It's much more efficient to just ask the osm layer
whether it is enabled, than to use D3 to find that layer in the DOM and check
whether it's classed `disabled`
This is more work to further isolate the layers that entities draw to.
It makes it easier to debug what is going on, and can eventually lead to
deferred drawing, if each draw function is in its own place and not dependant
on anything else.
I've started to replace the vertex-hover with an explicit layer for touch
targets.
Also had to change a lot of the svg tests, which are really brittle.
Things would happen like - the surface would be created, it would kick of a
deferred redraw, which would notice that the zoom was 0 and call
editOff, which would remove the osm layers that were just created and
that the tests were trying to draw to. These tests need proper zoom and
projection otherwise nothing works.
