performance enhancements

memory enhancements
prevent starting sensors more than once
fix for wifi lag issues
map scaling for huge buildings

sensors/android/AccelerometerSensorAndroid.h

@@ -23,6 +23,8 @@ private:
 		;
 	}
 
+	bool started = false;
+
 public:
 
 	/** singleton access */
@@ -33,26 +35,30 @@ public:
 
 	void start() override {
 
+		if (started) {return;}
+		started = true;
+
 		auto onSensorData = [&] () {
 			AccelerometerData data(acc.reading()->x(), acc.reading()->y(), acc.reading()->z());
 			informListeners(data);
 		};
 
+		// accelerometer is usually verry fast -> limit the maximum data-rate to 200 Hz (5ms)
+		acc.setDataRate(200);
+
 		acc.connect(&acc, &QAccelerometer::readingChanged, onSensorData);
 		acc.start();
 
 	}
 
 	bool isRunning() const override {
-		return acc.isActive();
+		return started;
 	}
 
 	void stop() override {
 		throw "TODO";
 	}
 
-
-
 };
 
 #endif // ANDROID