Retina support

This isn't quite ready to merge yet, but I'm doing this to get a place to discuss it. 
The major part that is missing it using SDL 2.0.1 for OS X. 
I have been having issues compiling the version in lime-build locally, but I'm not sure if it's my fault, limes fault or something wrong with that particular version of SDL. 

I am also not entirely sure how this will behave on Windows 8 which I hear has at least partial support for funky DPI stuff.