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.
Based on the below, m_iPos should remain in terms of the source rectangle -- or maybe we should just keep the parameter itself and do the calculations on the fly in the paintEvent.