Fix inherited parallax values causing layers to display incorrectly

[LilithSilver]

Fixes #3124.

Before, parallax was causing group layers to be offset as well as non-group layers. Now, only non-group layers have the parallax offset, so that they can properly use their effective inherited parallax value.

Visually tested on the scene I included in the issue. Also, tested with a 2-layer inheritance: 0.8 * 0.5 * 0.5 = 0.2.

[bjorn]

Good catch and nice fix!

Since the layer item position is actually set in three places (on creation, when updating all positions and when responding to changes in layer properties), I've introduced a helper function with the group layer check.

I considered whether it would be possible to use the local parallax offset instead of the effective one, but it seems difficult to obtain the same behavior that way. Could it be that the current behavior is not intuitive at all? Maybe parallax factors should be summed up instead of multiplied (after subtracting 1). I wonder if anybody would be relying on the multiplication behavior already...

[LilithSilver]

Sounds good, thank you!

Personally, I think multiplication is pretty intuitive. It's exactly how I would expect it to work -- something going slower in the background should go even slower if I made their parent go slower, and vice versa. I think where it gets confusing is with negative values -- a negative times a negative is a positive! -- but negative values aren't super useful, and the user can always make sure their group structure accounts for that. Or perhaps a multiplication strategy like -2 * -0.5 = - 1 is in order, for those; I'm not sure.

It's also pretty easy to implement with multiplication; I just added it to my renderer for my game and it really wasn't bad!

[bjorn]

@LilithSilver Thanks for sharing your thoughts! One thing that bothered me about the multiplication, is that once you put a group on factor 0, you can't make any of its child layers move, but I guess that's unlikely to bother anyone in practice. I think alternatively, it could behave like any value set on a child layer would override the value of the parent rather than multiply by it (but of course then we need to introduce an "unset" state).