Define a landing point relative to all markers

[kripper]

To avoid the bouncing between two markers that are switched intermittently active (green) and inactive (red), we should extend the configuration string to define a "landing point" = a virtual point that is not located on any given marker, but defined relative to all markers.

This way, we can also handle multiple active markers at the same time.

To implement this, we should define the (x,y) offset for every marker's center relative to the landing point.

@fnoop please show signs of live

[kripper]

See chobitsfan/apriltag_plnd#1

[kripper]

Implemented here: https://github.com/kripper/vision-landing-2/