Fixed OS X retina initial videomode size

OSX current_videomode size is now initialized with correct resolution based on backing store. Fixes issue where content does not correctly fit in windows on retina displays when first drawn (before this change, it was necessary to resize a window to get the videomode size to be correct). This issue was prevalent on my 15in MBPr 2012, running Mavericks 10.9.1, on the discrete card.

@solgar posted an image of what this fixes in issue #20.