Fixedwing position controller not able follow position setpoints in offboard mode

[Jaeyoung-Lim]

Describe the bug
When streaming position setpoints to the fixed wing vehicle, it can enter offboard mode as the setpoint is sreamed. However, rather than following the setpoints (usually defined in local coordinates) it simply flies straight.

This seems to be because the fixed wing position controller blindly reads from the setpoints in global coordinates from the position_setpoint_triplets.

https://github.com/PX4/Firmware/blob/master/src/modules/fw_pos_control_l1/FixedwingPositionControl.cpp#L854-L855

To Reproduce

1. Run SITL and start mission while streaming position_setpoints through mavros with setpoint_raw messages to takeoff and loiter
2. After takeoff, switch vehicle to offboard mode

Expected behavior
When given a position setpoint, the desired behavior is to use it as a reference for the L1 position controller.

Log Files and Screenshots
https://review.px4.io/plot_app?log=f6204199-f876-4253-8d12-cb454c1e816b

As can be seen in the picture below, once the vehicle is in offboard mode, the vehicle simply flies straight.

Additional context
Attitude setpoints for offboard mode in fixed wing is currently working. However, to use this a l1 like path tracking controller needs to be implemented on the companion side to make it usable. Being able to stream position setpoints to a fixed wing drone would greatly simplify the interface for robotic applications in fixed wing