core/colorpipeline: only reduce the reference luminance enough to get 50% headroom

If the output side / display can already show at least 50% headroom, dimming the image to get
more space for the highlights doesn't improve the highlights a lot, definitely not enough to
justify making the rest of the image worse for it.

src/core/colorpipeline.cpp

@@ -343,7 +343,7 @@ ColorTonemapper::ColorTonemapper(double referenceLuminance, double maxInputLumin
     m_inputRange = maxInputLuminance / referenceLuminance;
     const double outputRange = maxOutputLuminance / referenceLuminance;
     // = how much dynamic range this algorithm adds, by reducing the reference luminance
-    m_addedRange = std::clamp(m_inputRange / outputRange, 1.0, maxAddedHeadroom);
+    m_addedRange = std::min(m_inputRange, std::clamp(maxAddedHeadroom / outputRange, 1.0, maxAddedHeadroom));
     m_outputReferenceLuminance = referenceLuminance / m_addedRange;
 }
 

src/opengl/colormanagement.glsl

@@ -117,7 +117,7 @@ vec3 doTonemapping(vec3 color) {
     // if the reference is too close to the maximum luminance, reduce it to get up to 50% headroom
     float inputRange = maxTonemappingLuminance / destinationReferenceLuminance;
     float outputRange = maxDestinationLuminance / destinationReferenceLuminance;
-    float addedRange = min(inputRange / outputRange, 1.5);
+    float addedRange = min(inputRange, clamp(1.5 / outputRange, 1.0, 1.5));
     float outputReferenceLuminance = destinationReferenceLuminance / addedRange;
 
     // keep it linear up to the reference luminance