Previously, SDL would always expose display modes and window dimensions in terms of pixels, and would add an extra 'fake' display mode on retina screens which would contain the non-retina resolution. Calling SDL_CreateWindow with the dimensions of that fake display mode would not work.
Now, SDL only exposes display modes and window dimensions in terms of points rather than pixels. If the SDL_WINDOW_ALLOW_HIGHDPI flag is passed into SDL_CreateWindow, then any OpenGL contexts created from that window will be sized in pixels rather than points (retrievable with SDL_GL_GetDrawableSize.) Window dimensions and mouse coordinates are still in terms of points rather than pixels even with that flag.
This matches the behavior of SDL in OS X more closely, and lets users choose whether to make use of retina displays and lets them handle it properly.
