Don't clear overlay on mouseup if not zooming.

Don't clear overlay on mouseup if not zooming.

On endZoom, check if moveZoom was called since the last startZoom. If
not, leave the overlay canvas alone, there's no zoom rectangle to erase.

The default mouseup action unconditionally cleared the overlay canvas,
this destroyed other things that may have been drawn there such as
highlighted series, causing glitches for clickCallback use.