Servo control motion actuator did not work as expected when the object
is moving on a moving platform.
This patch introduces a new Ref field in the servo motion actuator
to set a reference object for the velocity calculation.
You can set the object during the game using the actuator "reference"
attribute; use an object name or an object reference.
The servo controller takes into account the angular velocity of the
reference object to compute the relative local velocity.
07fc2aa526