message: Fix undefined behavior in line drawing.
  Left shift of negative values is undefined.
diff: @@ -655,7 +655,7 @@ Line2( Mat& img, Point2l pt1, Point2l pt2, const void* color)
  pt1.y ^= pt2.y & j;
  x_step = XY_ONE;
  - y_step = (dy << XY_SHIFT) / (ax | 1);
  + y_step = dy * (1 << XY_SHIFT) / (ax | 1);
  ecount = (int)((pt2.x - pt1.x) >> XY_SHIFT);
  }
  else
  @@ -668,7 +668,7 @@ Line2( Mat& img, Point2l pt1, Point2l pt2, const void* color)
  pt2.y ^= pt1.y & i;
  pt1.y ^= pt2.y & i;
  - x_step = (dx << XY_SHIFT) / (ay | 1);
  + x_step = dx * (1 << XY_SHIFT) / (ay | 1);
  y_step = XY_ONE;
  ecount = (int)((pt2.y - pt1.y) >> XY_SHIFT);
  }
  