Fixing window positioning bug in Mac OS X 

by moving the call to SDL_GetWindowPosition before SDL_SetWindowSize since OS X windows resize from the bottom left corner (not the top left as in Windows) which means that resizing the window will push the top edge up higher, but SDL_GetWindowPosition returns the top-left corner of the window, which has now shifted up. So we have to query the position prior to resizing if we want the proper previous location values.

This fixes both the default window position (which was wrong if you set anything other than the default resolution) as well as the position of the window when resizing it programmatically.