avoid int overflow in task drawing using clipping
  - it's also avoid to draw not visible parts of task and reduce cpu overhead.
fix int overflow in topology at hight zoom level ( <15m with france_full.lkm )
fix integer overflow in irotate(...)
 ( used in screen coordinate calculation of waypoint )
add INT_OVERFLOW=y for debuging int overflow
Merge branch 'InOverflow' of http://github.com/brunotl/LK8000 into bruno