Check the cost of the center point of the robot and return if it's non-traversable space

[corot]

AB#18500

when there's a small obstacle right in front of the robot and obstacle_approach_distance is significant, the enlarged footprint can engulf the obstacle and so the robot is not considered in collision:

PF_fp_KO.mp4

This cannot happen if we check also the cell at the center of the robot, as it will enter inflated space as soon as we start moving:

PF_fp_OK.mp4

Other algorithms check the fp cost the same way, e.g. Hybrid A*.
In theory this should also provide a small increase in performance, as checking a cell is much faster than the checking the footprint lines

[plsang]

the change is reasonable to me