fix(ekf2): allow optical flow to start when range finder is height reference

When EKF2_HGT_REF=2 (range sensor) with no GPS, optical flow could
never start. The starting condition required isTerrainEstimateValid()
or isHorizontalAidingActive(), but terrain is never "estimated" when
range is the height reference (ground is the datum, terrain state is
fixed at 0), and there's no horizontal aiding without GPS.

HAGL is directly known from the range measurement in this case, so
optical flow has everything it needs to fuse. Add the range height
reference check to the optical flow starting conditions.

Fixes: #25248

Author: dakejahl

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

@@ -155,7 +155,7 @@ void Ekf::controlOpticalFlowFusion(const imuSample &imu_delayed)
 				&& is_magnitude_good
 				&& is_tilt_good
 				&& (_flow_counter > 10)
-				&& (isTerrainEstimateValid() || isHorizontalAidingActive())
+				&& (isTerrainEstimateValid() || isHorizontalAidingActive() || (_height_sensor_ref == HeightSensor::RANGE))
 				&& 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.