Fix Qt6 signature change for nativeEventFilter (#7254)

* Adds win32EventFilter a wrapper for nativeEventFilter on Windows
* win32EventFilter is currently used to intercept top-level Window events (currently, to avoid VSTs setting transparency of the parent application)

include/MainApplication.h

@@ -49,10 +49,14 @@ class MainApplication : public QApplication
 	MainApplication(int& argc, char** argv);
 	bool event(QEvent* event) override;
 #ifdef LMMS_BUILD_WIN32
-	bool winEventFilter(MSG* msg, long* result);
-	bool nativeEventFilter(const QByteArray& eventType, void* message,
-				long* result);
-#endif
+#if (QT_VERSION < QT_VERSION_CHECK(6,0,0))
+	using FilterResult = long;
+#else
+	using FilterResult = qintptr;
+#endif // QT6 check
+	bool win32EventFilter(MSG* msg, FilterResult* result);
+	bool nativeEventFilter(const QByteArray& eventType, void* message, FilterResult* result);
+#endif // LMMS_BUILD_WIN32
 	inline QString& queuedFile()
 	{
 	    return m_queuedFile;

src/gui/MainApplication.cpp

@@ -86,8 +86,8 @@ bool MainApplication::event(QEvent* event)
 }
 
 #ifdef LMMS_BUILD_WIN32
-// This can be moved into nativeEventFilter once Qt4 support has been dropped
-bool MainApplication::winEventFilter(MSG* msg, long* result)
+// Helper function for nativeEventFilter
+bool MainApplication::win32EventFilter(MSG* msg, FilterResult* result)
 {
 	switch(msg->message)
 	{
@@ -110,11 +110,11 @@ bool MainApplication::winEventFilter(MSG* msg, long* result)
 }
 
 bool MainApplication::nativeEventFilter(const QByteArray& eventType,
-					void* message, long* result)
+					void* message, FilterResult* result)
 {
 	if(eventType == "windows_generic_MSG")
 	{
-		return winEventFilter(static_cast<MSG *>(message), result);
+		return win32EventFilter(static_cast<MSG*>(message), result);
 	}
 	return false;
 }