Refine retina versions of controls

[tmcw]

This should be _the_push to make things perfectly retina & mobile compatible.

• DONE: zoom controls should adapt or look correct at 1:1 px
• interaction controls may need the fix in Thinking about touch events for interaction on high DPI displays cutting-room-floor/wax#198
• should users be able to supply a retina version of their tiles?

[tmcw]

Removing from milestone: retina versions of tiles and the retina tweak for interaction will come in the next push.

[tristen]

Sorry I should have responded to this one:

DONE: zoom controls should adapt or look correct at 1:1 px

I didn't do any work here - isn't this a TODO?

[tmcw]

zoom controls look 'fine' since the pixels are mostly doubled fine, but now I'm realizing that fullscreen etc will need a boost