SlideFromBelow/SlideFromAbove don't animate & get stuck in original, non-centered position

Worked in 0.31. No longer working in 0.3.2

Looked at diffs.  It looks like the following if statement was removed from the ShowModal function, when either SlideFromBelow or SlideFromAbove options are true.

```
if (coords.isEmpty()) {
 animateProperties.top = positionY;
} else {
 animateProperties.top = coords.top;
}
```

When I add this condition back, animation works again.
