VuMeter bar to use full widget size

Check widget size to know bar size instead of checking bitmap size to allow resizable VuMeters.
Also suddenly found fix for unavailable horizontal VuMeter peaks.
P.S. I don't know C++ at all, so code may be "incorrenct", but it works with all skins, just tested.
I think sourceRect needs to remain the dimensions of the pixmap. targetRect (doesn't exist yet) needs to be the full size of the widget. That way the Paintable will either stretch or tile based on the setting.