ekf2-flow: only allow flow when in range

Also, as the flow makes the link between range and horizontal velocity,
only allow it to start if at least one of the two is known. Otherwise
the EKF will struggle to estimate both values at the same time.

src/modules/ekf2/EKF/aid_sources/optical_flow/optical_flow_control.cpp

@@ -118,16 +118,17 @@ void Ekf::controlOpticalFlowFusion(const imuSample &imu_delayed)
 					&& (_control_status.flags.inertial_dead_reckoning // is doing inertial dead-reckoning so must constrain drift urgently
 					    || isOnlyActiveSourceOfHorizontalAiding(_control_status.flags.opt_flow));
 
-		const bool is_within_max_sensor_dist = getHagl() <= _flow_max_distance;
+		const bool is_within_sensor_dist = (getHagl() >= _flow_min_distance) && (getHagl() <= _flow_max_distance);
 
 		const bool continuing_conditions_passing = (_params.flow_ctrl == 1)
 				&& _control_status.flags.tilt_align
-				&& is_within_max_sensor_dist;
+				&& is_within_sensor_dist;
 
 		const bool starting_conditions_passing = continuing_conditions_passing
 				&& is_quality_good
 				&& is_magnitude_good
 				&& is_tilt_good
+				&& (isTerrainEstimateValid() || isHorizontalAidingActive())
 				&& isTimedOut(_aid_src_optical_flow.time_last_fuse, (uint64_t)2e6); // Prevent rapid switching
 
 		// If the height is relative to the ground, terrain height cannot be observed.