Fix behavior of DitheringMode with QVariant in Qt5

Fixes #3304

src/core/Dithering.hpp

@@ -29,6 +29,14 @@ namespace ForTextureMgr
 GLuint makeDitherPatternTexture(QOpenGLFunctions& gl);
 }
 
+// This namespace is only needed to add the Q_NAMESPACE and Q_ENUM_NS for Qt5
+// to be able to store DitheringMode in QVariant and convert its values to
+// string. Normally (e.g. in Qt6) I'd just put the enum into the global
+// namespace.
+namespace Dithering
+{
+Q_NAMESPACE
+
 enum class DitheringMode
 {
 	Disabled,    //!< Dithering disabled, will leave the infamous color bands
@@ -37,6 +45,9 @@ enum class DitheringMode
 	Color888,    //!< 24-bit color (AKA True color)
 	Color101010, //!< 30-bit color (AKA Deep color)
 };
+Q_ENUM_NS(DitheringMode)
+}
+using Dithering::DitheringMode;
 
 Vec3f calcRGBMaxValue(DitheringMode mode);