SCUMM V1-V2: Fix actor walk behavior (transitions between walk boxes, and changing destination issue)

Two issues have been corrected,

1. When an actor reached the edge of a walk box they would 'walk on the spot' for 2 steps
2. When the walk destination was changed, the actor would not change destination until the current destination was reached, or the edge of a walk box was encountered. 

These fixes are based on the MM V2 Dos Demo executable (it wasn't packed :), but Zak/MM (inc NES) all appear to be working correctly now (don't have the FMTowns version, so am unable to test it)