Before, if you want to see the FPS, you need to close the client and
restart it with the -k switch to see the FPS. This is annoying.
This PR introduces a new keybind, ScrollLock+D, which, when pressed,
toggles the display of the FPS.
This is implemented for both EGL and OpenGL backends.
As the window manager may change our mode to full screen without our
request we must ask the ds backend for the current state when we want to
toggle the mode.
