filter: don't allow an accel factor of 0 on the flat profile
Leave a narrow gap so the mouse moves excruciatingly slow instead of not moving at all. This allows to recover from overexcited mouse speed slider movements. https://bugs.freedesktop.org/show_bug.cgi?id=102501 Signed-off-by:Peter Hutterer <peter.hutterer@who-t.net>