core/colorspace: match default luminances with the Wayland protocol

It wasn't feasible to do it before, because it made SDR black far too bright, but
with black point compensation the result looks as users expect it

autotests/test_colorspaces.cpp

@@ -141,15 +141,15 @@ void TestColorspaces::testColorPipeline_data()
     QTest::addRow("sRGB -> rec.2020 relative colorimetric")
         << ColorDescription(NamedColorimetry::BT709, TransferFunction(TransferFunction::gamma22), TransferFunction::defaultReferenceLuminanceFor(TransferFunction::gamma22), 0, std::nullopt, std::nullopt)
         << ColorDescription(NamedColorimetry::BT2020, TransferFunction(TransferFunction::PerceptualQuantizer), 500, 0, std::nullopt, std::nullopt)
-        << QVector3D(0.044, 0.044, 0.044)
-        << QVector3D(0.517, 0.517, 0.517)
-        << QVector3D(0.677, 0.677, 0.677)
+        << QVector3D(0.06729, 0.06729, 0.06729)
+        << QVector3D(0.51667, 0.51667, 0.51667)
+        << QVector3D(0.67658, 0.67658, 0.67658)
         << RenderingIntent::RelativeColorimetric;
     QTest::addRow("sRGB -> scRGB relative colorimetric")
         << ColorDescription(NamedColorimetry::BT709, TransferFunction(TransferFunction::gamma22), TransferFunction::defaultReferenceLuminanceFor(TransferFunction::gamma22), 0, std::nullopt, std::nullopt)
         << ColorDescription(NamedColorimetry::BT709, TransferFunction(TransferFunction::linear, 0, 80), 80, 0, std::nullopt, std::nullopt)
-        << QVector3D(0.0001, 0.0001, 0.0001)
-        << QVector3D(0.2177376408240310, 0.2177376408240310, 0.2177376408240310)
+        << QVector3D(0.00025, 0.00025, 0.00025)
+        << QVector3D(0.217833, 0.217833, 0.217833)
         << QVector3D(1, 1, 1)
         << RenderingIntent::RelativeColorimetric;
 }

src/core/colorspace.cpp

@@ -377,7 +377,7 @@ double TransferFunction::defaultMinLuminanceFor(Type type)
     switch (type) {
     case Type::sRGB:
     case Type::gamma22:
-        return 0.01;
+        return 0.02;
     case Type::linear:
         return 0;
     case Type::PerceptualQuantizer:
@@ -391,7 +391,7 @@ double TransferFunction::defaultMaxLuminanceFor(Type type)
     switch (type) {
     case Type::sRGB:
     case Type::gamma22:
-        return 100;
+        return 80;
     case Type::linear:
         return 1;
     case Type::PerceptualQuantizer:
@@ -409,7 +409,7 @@ double TransferFunction::defaultReferenceLuminanceFor(Type type)
         return 80;
     case Type::sRGB:
     case Type::gamma22:
-        return 100;
+        return 80;
     }
     Q_UNREACHABLE();
 }