Prevent Parallax2D autoscroll reset

[markdibarry]

Pointed out by @Mickeon, autoscroll currently resets the offset to 0,0 every time it's set. This is undesirable, because users may want to update this value on the fly to speed up or slowdown the scroll. The current behavior may be needed in some cases, so rather than exposing the autoscroll_offset to be able to update it manually, we remove it, and use scroll_offset internally instead.

Also, apparently Math::fposmod(0,0) returns NaN, it just wasn't a problem until now, so a check was added in those cases.

[Mickeon]

There's a good chunk of "offsets" to think about with Parallax2D, and this PR removes one of them, which was previously only available internally.
Seeing the scroll_offset changing in the Remote Tree instead of some arbitrary internal value is considerably more intuitive. It helps implementing custom scrolling behavior, as well.

This breaks compatibility but that's exactly why Parallax2D is experimental.

[akien-mga]

This breaks compatibility but that's exactly why Parallax2D is experimental.

I don't think this breaks compatibility, at least in the usual sense of breaking the API (adding/change/removing methods/arguments from the public API). Changing behavior of an API is the natural consequence of any bugfix so it's not considered a compat breakage.

[akien-mga]

Thanks!

[SelfDevTV]

I found a workaround in the meantime for 4.3

// will stop the Autoscroll (or incrase / descrease it) and 
// will save the position it was at and put it back, so it transitions smoothly. 

Vector2 pos = p.Position;
Autoscroll = Vector2.Zero;
Position = pos;