* FlightTask: Fix ekf2 reset race condition during task switch
During a loss of GPS data when using GPS as primary height source,
the height is reset to baro and the local position gets invalid at the
same time. This triggers a switch to altitude flight task and a setpoint
reset.
This combination of events had the effect to ignore the height reset,
the large sudden height error could create an abrupt change of altitude
or even a crash.
The ekf2 reset is now done at the beginning of each update call.
Adjusting the tilt limit can lead to diverging position control
and should only be used by setting a sanity limit for the controller
and not to adjust during the descent phase of a Land or RTL.
Otherwise it leads to flyaways in important failsafe modes when
there's stronger disturbance e.g. wind.
The modified statements are intended to
1. Set a new Z position setpoint that is equivalent to the current distance
to ground setpoint,
and
2. Set a new distance to ground setpoint that is equivalent to the current Z
position setpoint.
They are only called in terrain hold mode, when activating/deactivating the
holding (typically when coming to a stop and when starting to move again).
The setpoints take the current control error into account, but because the
control error is added, not subtracted, the result is that the new setpoint
is 2 times the control error off from the old setpoint, instead of being
at the same spot as the old setpoint.
This commit temporarily breaks direct horizontal thrust setpoint execution
which is used by FlightTaskManualAltitude. This is necessary to allow for
PositionControl cleanup namely calculating the whole velocity PID in one
Vector3f formula.
Having this in a separate commit is useful since it reduces indentation
of a bigger code block.
