The GLFW windowing backend supports window-resizing, so I don't think
it's wrong to add enhancements to the backend code. It's not like the
SDL2 backend emulates DirectDraw's unstable framerate or anything.
Now, instead of keyboard input being completely-handled in the
backend, it's merely abstracted, while still otherwise being handled
in the game itself. This will be useful for the enhanced branch's
key-rebinding menu.
