Provide a workaround if SDL_GetRelativeMouseState() is disfunctional

On some systems SDL_SetRelativeMouseMode() can fail under several
circumstances. For example:
 - Security software is blocking acces to raw input.
 - XInput 2 is unavailable.
 - SDL2s configure script has messed up.
Detect those situations and reposition the mouse manually to the
window center after having read it's current state. This fixes
issue #34 an #35.