[anotherworld] vertical scrolling implementation is wrong

[felipesanches]

Vertical scrolling is implemented via an attribute to the copyPage video command.

It is used for shaking the screen (conveying the sense of an earthquake) in the first level of the game. In that level, the current implementation seems to work properly.

It is also used on the elevator scene. But in that scene I get the impression that the scrolling is reversed (scrolling in the wrong direction). I must observe, though, that this may not actually be a scrolling issue, as the background rendering seems corrupted upon first screen loading even before triggering vertical scrolls by going up & down in the elevator. So this may be a side-effect of some other screen rendering issue, instead.

[felipesanches]

After fixing #1 I can tell that the scrolling is certainly being performed in the opposite direction.

[felipesanches]

I've inverted the direction of the scrolling, and some static backgrounds look good now. But scrolling animations are still wrong.