QuadPlane: QRTL moves away from home if vehicle took off before getting GPS lock

I've tested QuadPlane with master (4th Apr 2018) and noticed this odd behaviour if the user takes off before the EKF sets the origin.

- when QRTL is initiated the vehicle originally flies to the takeoff location but then as it descends it moves to another location (perhaps the location that the vehicle was at when the EKF origin was set
- during the final phase of QRTL, the vehicle will sometimes stop descending a meter or two off the ground

Reproduce QuadPlane QRTL after takeoff before GPS lock or EKF origin initialization. Trace the QRTL return and final-descent behavior, since the issue names no files or tests. Done means the vehicle continues toward the intended home or takeoff location and completes the final descent at ground level.

Written by the indexing model from the issue text.