Fix regression in rendering the Window-Motion-Outline

Commit 79fe2fca1de5140f538e68f6981b27cf7f917e7a checks for pending
motion events and drops out of the FluxboxWindow::motionNotifyEvent() function
early if so. When the user does not use the opaque window movement method an
outline will be drawn to the screen. That outline was not cleaned correctly
with commit 79..