When the compositor is stopped there might still be a buffer swap ongoing, in
particular when a client blocks compositing on X11.
Depending on the backend the next buffer swap event might be handled in
bufferSwapComplete (Wayland) or not be handled (X11 GLX, since a new
GLX window will be created while the swap event is sent for the old one).
With this patch the buffer swap state is reset on stop such that on later
start no outdated data might create errors and instead a new repaint can be
triggered with updated data.