barLevel: Don't show border radius if the value is 0
Fix a known regression from  that caused the volume bar in the OSD window to never be hidden, even if the volume is set to 0. This happened because the border radius of the barLevel is always drawn without ensuring that the actual bar is visible.
To fix this, simply check if the value to draw is 0, and if it is, don't draw the border radius of the bar at all.
 !2 (merged)