The sequencer has a feature where it only draws a preview while scrubbing.
This was introduced in 8abdc89912.
When scrubbing ends, the sequencer requires an extra update so that the
non-preview version is shown again.
This is only needed by the sequencer and hence the update should not be
done if the sequencer is not used.
Changing `screen->scrubbing` is moved out of the sequencer specific
functions, because it's used elsewhere as well.
037b3f87bd