Used by the routines at UpdateLeech, UpdateMidges and UpdateSlug.

The checks which are made at the start of the update of every bug. This counts the bug (the bug counter at CurrentBug_ID is used to decide when to spawn more bugs), remembers the position and state of the bug so that it can be erased, checks the bug against the leaves and finally checks whether the bug is touching Robbie by jumping to BugCollision, which returns to the caller.