UI: Do not allow mouse wheel for volume slider

This commit fixes the issue of being able to scroll through the mixer
with the mouse wheel without modifying the volume slider.  This
functionality can be temporarily restored by holding a click on the
volume meter.

UI/volume-control.hpp

@@ -198,6 +198,9 @@ public:
 	qreal getInputPeakHoldDuration() const;
 	void setInputPeakHoldDuration(qreal v);
 	void setPeakMeterType(enum obs_peak_meter_type peakMeterType);
+	virtual void mousePressEvent(QMouseEvent *event) override;
+	virtual void wheelEvent(QWheelEvent *event) override;
+	virtual void leaveEvent(QEvent *event) override;
 
 protected:
 	void paintEvent(QPaintEvent *event) override;