Introduce range with solid warn color

Introduce a second point in the gradient for the warn colors so that we get a certain range with the full/solid warn color.

The colors are distributed as follows now. The solid ok range goes from -inf dbFS to -12 dbFS. The warn range goes from -6 dbFS to 0 dbFS. In between the colors are interpolated. Values above 0 dbFS interpolate from the warn color to the clip color.

This is now quite similar to the previous implementation.

# Analysis of the previous pixmap implementation
The pixmap implementation used pixmaps with a height of 116 pixels to map 51 dbFS (-42 dbFS to 9 dbFS) across the whole height. The pixels of the LED pixmap were distributed as follows along the Y-axis:
* Margin: 4
* Red: 18
* Yellow: 14
* Green: 76
* Margin: 4

Due to the margins the actual red, yellow and green areas only represent a range of (1 - (4+4) / 116) * 51 ~ 47,48 dbFS. This range is distributed as follows across the colors:
Red: 7.91 dbFS
Yellow: 6.16 dbFS
Green: 33.41 dbFS

The borders between the colors are located along the following dbFS values:
* Red/yellow: 9 - (4 + 18) / 116 * 51 dbFS ~ -0.67 dbFS
* Yellow/green: 9 - (4 + 18 + 14) / 116 * 51 dbFS ~ -6.83 dbFS
* The green marker is rendered for values above -40.24 dbFS.

src/gui/widgets/Fader.cpp

@@ -388,7 +388,9 @@ void Fader::paintLevels(QPaintEvent * ev, QPainter & painter, bool linear)
 	float const mappedPersistentPeakL(mapper(m_persistentPeak_L));
 	float const mappedPersistentPeakR(mapper(m_persistentPeak_R));
 	float const mappedUnity(mapper(1.f));
-	float const mappedLastOk(mapper(dbfsToAmp(-12.f)));
+	float const mappedWarnEnd(mappedUnity);
+	float const mappedWarnStart(mapper(dbfsToAmp(-6.f)));
+	float const mappedOkEnd(mapper(dbfsToAmp(-12.f)));
 
 	PaintHelper ph(mappedMinPeak, mappedMaxPeak);
 
@@ -399,8 +401,9 @@ void Fader::paintLevels(QPaintEvent * ev, QPainter & painter, bool linear)
 
 	QLinearGradient linearGrad(0, margin, 0, leftMeterRect.y() + leftMeterRect.height());
 	linearGrad.setColorAt(0, clippingColor);
-	linearGrad.setColorAt(ph.mapMaxZeroAndMinOne(mappedUnity), warnColor);
-	linearGrad.setColorAt(ph.mapMaxZeroAndMinOne(mappedLastOk), okColor);
+	linearGrad.setColorAt(ph.mapMaxZeroAndMinOne(mappedWarnEnd), warnColor);
+	linearGrad.setColorAt(ph.mapMaxZeroAndMinOne(mappedWarnStart), warnColor);
+	linearGrad.setColorAt(ph.mapMaxZeroAndMinOne(mappedOkEnd), okColor);
 	linearGrad.setColorAt(1, okColor);
 
 	// Draw left levels