Fix marker code for OS X (UIImage vs NSImage) about mbxmapkit

So here's an interesting bit of additional complexity... http://supermegaultragroovy.com/2012/10/24/coding-for-high-resolution-on-os-x-read-this/

For deciding the correct resolution at which to request and display marker icons on OS X, something to consider is, what happens if your map app is launched on the built in screen of a Retina MacBook, then somebody drags it over to a low dpi external monitor?

For now I'm going to take the approach of, "Just make sure it compiles and looks okay on a 1x screen".

[incanus]

Interesting. Chris is right that what flies on iOS isn't necessarily kosher here.

Seems like implementing -[NSView viewDidChangeBackingProperties] is the way to go eventually, but I think a good progression for this is:

1. Work well on 1x, as you say, which closes this ticket and hits our milestone.
2. Support 2x retina displays on OS X.
3. Dynamically change between them per above method. Should be able to drag the app window back and forth and get markers sizing properly, though possibly with a delay as they fetch either version for the first time.

In fact, map tiles technically need reloading, too, so this is a whole other ball of wax. I wouldn't put it too high on the list though as OS X is a "nice to have because MapKit now works there" but not a huge Mapbox focus, per se.

Ticket-wise I would hit the first bullet on this one, then make two more tickets for the others.

Yeah, the goal I'm aiming for here is "compiles, links, and runs without any critical errors"