Moving: Only set sfDirty flag if velocity is really used.
Confine bounding boxes and y-distance for flag collection.

This leads to less blocks visited. (Cleanup pending.)
Don't check god mode for dead players.
Regroup lost-ground workarounds. Fix some aspects.

Check some preconditions in the check method, delegate to different
methods for different cases, also to have smaller method bodies.

IsAboveStairs is only checked for "mild" y-distances. For faster
descending an individual edge-check has been added.
Let NoFall be more generous with the on-ground search.

Extend xz and y margins.