core/colorspace: ensure that we don't create elevated blacks with black point compensation

There are cases where the minimum luminance can be lower than the transfer function's minimum; in those
cases, the transfer function's minimum would get mapped to a value greater than black in the target
transfer function. To avoid that, this just takes the max. of the metadata and transfer function minimum

BUG: 494854
(cherry picked from commit cef6656ff5)

autotests/test_colorspaces.cpp

@@ -225,7 +225,7 @@ void TestColorspaces::testOpenglShader_data()
     QTest::addColumn<double>("maxError");
 
     // the allowed error here needs to be this high because of llvmpipe. With real GPU drivers it's lower
-    QTest::addRow("Perceptual") << RenderingIntent::Perceptual << 6.5;
+    QTest::addRow("Perceptual") << RenderingIntent::Perceptual << 7.0;
     QTest::addRow("RelativeColorimetric") << RenderingIntent::RelativeColorimetric << 1.5;
     QTest::addRow("AbsoluteColorimetric") << RenderingIntent::AbsoluteColorimetric << 1.5;
     QTest::addRow("RelativeColorimetricWithBPC") << RenderingIntent::RelativeColorimetricWithBPC << 1.5;

src/core/colorspace.cpp

@@ -461,11 +461,14 @@ QMatrix4x4 ColorDescription::toOther(const ColorDescription &other, RenderingInt
         // add black point compensation: black and reference white from the source color space
         // should both be mapped to black and reference white in the destination color space
 
+        const double effectiveMin = std::max(minLuminance(), m_transferFunction.minLuminance);
+        const double otherEffectiveMin = std::max(other.minLuminance(), other.m_transferFunction.minLuminance);
+
         // before color conversions, map [src min, src ref] to [0, 1]
         luminanceBefore.scale(1.0 / (reference - minLuminance()));
-        luminanceBefore.translate(-minLuminance(), -minLuminance(), -minLuminance());
+        luminanceBefore.translate(-effectiveMin, -effectiveMin, -effectiveMin);
         // afterwards, map [0, 1] again to [dst min, dst ref]
-        luminanceAfter.translate(other.minLuminance(), other.minLuminance(), other.minLuminance());
+        luminanceAfter.translate(otherEffectiveMin, otherEffectiveMin, otherEffectiveMin);
         luminanceAfter.scale(other.referenceLuminance() - other.minLuminance());
     } else {
         // map only the reference luminance