core/log: crash if Quickshell's log filter is installed twice

Crashes from recursion inside filterCategories through the old filter
have been observed. Presumably this means the log filter is getting
installed twice somehow. This should catch it.

Author: outfoxxed

src/core/logging.cpp

@@ -310,10 +310,15 @@ void LogManager::init(
 		instance->rules->append(parser.rules());
 	}
 
-	qInstallMessageHandler(&LogManager::messageHandler);
-
 	instance->lastCategoryFilter = QLoggingCategory::installFilter(&LogManager::filterCategory);
 
+	if (instance->lastCategoryFilter == &LogManager::filterCategory) {
+		qCFatal(logLogging) << "Quickshell's log filter has been installed twice. This is a bug.";
+		instance->lastCategoryFilter = nullptr;
+	}
+
+	qInstallMessageHandler(&LogManager::messageHandler);
+
 	qCDebug(logLogging) << "Creating offthread logger...";
 	auto* thread = new QThread();
 	instance->threadProxy.moveToThread(thread);