ekf2-flow: use same measurement prediction as in jacobian derivation

Also avoid double division in flow prediction

src/modules/ekf2/EKF/ekf.h

@@ -791,6 +791,7 @@ private:
 	// calculate optical flow body angular rate compensation
 	void calcOptFlowBodyRateComp(const flowSample &flow_sample);
 
+	float predictFlowHagl() const;
 	float predictFlowRange() const;
 	Vector2f predictFlow(const Vector3f &flow_gyro) const;