Non-standard fullscreen resolutions don't work properly on OSX

[rxi]

Making an issue so I remember to look into this; see the comments of #6

[rxi]

Tried looking into this but still unsure why it isn't working as expected. According to the SDL docs abnormal resolutions should be automatically adjusted for and work fine:

SDL will attempt to use a fullscreen mode. If a hardware resolution change is not possible (for whatever reason), the next higher resolution will be used and the display window centered on a black background.

This seems to work as expected on the the other platforms, so unless I've missed something I might have to assume this is an issue with either OSX or SDL and add a work around instead.