Removed giant block of commented code
Disabled init of SDL_TIMER as it was causing a crash on my system, seems to work fine without it. Not sure if this is needed?
Added support for retina displays

Makes liberal use of SDL_GL_GetDrawableSize to find out what the actual size of the window is.
Compensates for scaled mouse coordinates.
Had to switch to SDL_WINDOW_FULLSCREEN_DESKTOP to get things to work, unsure why.
Added support for retina DPI detection