- Clamp ME range to -64..63 (prevents corruption when me_range is too high)
- Allow MV's up to *and including* the positive range limit
- Allow out-of-edge ME by padding the prev buffer with a border of 0's
- Try previous MV before checking the rest (improves speed in some cases)
- More robust logic in code - ensure *mx,*my,*xored are updated together