Fix to allow filter correction with vision position estimate

When I tested the vision_estimate branch with sending vision_position_estimate mavlink messages I noticed the returned local_position_ned position wasn't the same. It appears it wasn't doing the filter correction.

https://github.com/PX4/Firmware/pull/1041

This is my first pull request so hopefully I am doing it correctly.

Also I guess the vision code added was just a quick hack to get the vision_position_estimate to work and will probably need to be written with its own bool use_vision_xy etc.?