Animations are run off the repaint cycle so if there's nothing to
repaint, an animation will stop running. This is usually not a problem
as each frame function of an animation causes something to change and
therefore a repaint to happen. This patch helps detect when the
animation isn't in said case and triggers a repaint to keep the
animation running.

This problem was found by using weston_move_scale_run() to move a view
onscreen from completely off. The very first time the animation frame
function was called the progress wasn't enough to move it into
view. The compositor saw there was nothing to repaint and stopped
doing anything else. When something else (like a pointer move) forced
a redraw, the view's position was very much onscreen and jumped into
view in an ugly way.