More flexible painting of fader ticks

Paint the fader ticks in a more systematic and flexible way. This also
removes some "magic numbers", e.g. by using `c_faderMinDb` instead of
`-120.f` as the lower limit.

The upper limit, i.e. the "starting point" is now also computed using
the maximum value of the model so that the fader will still paint
correctly if it ever changes.

Author: michaelgregorius

src/gui/widgets/Fader.cpp

@@ -706,7 +706,15 @@ void Fader::paintFaderTicks(QPainter& painter)
 	const QPen zeroPen(QColor(255, 255, 255, 216), 1.5);
 	const QPen nonZeroPen(QColor(255, 255, 255, 128), 1.);
 
-	for (float i = 6.f; i >= -120.f; i-= 6.f)
+	// We use the maximum dB value of the model to calculate the nearest multiple
+	// of the step size that we use to paint the ticks so that we know the start point.
+	// This code will paint ticks with steps that are defined by the step size around
+	// the 0 dB marker.
+	const auto maxDB = ampToDbfs(model()->maxValue());
+	const auto stepSize = 6.f;
+	const auto startValue = std::floor(maxDB / stepSize) * stepSize;
+
+	for (float i = startValue; i >= c_faderMinDb; i-= stepSize)
 	{
 		const auto scaledRatio = computeScaledRatio(i);
 		const auto maxHeight = height() - (height() - m_knob.height()) * scaledRatio - (m_knob.height() / 2);