Add Parallax Origin property to Map Properties

[krukai]

Independent from the plans for viewports, giving the user means to change the parallax offset (as in a "starting position") would help greatly to accommodate differences between Tiled's approach and that of a user's game project.

Please note that this is still WIP; the PR was created for enabling discussion (and bug fixing).

[eishiya]

Rather than "parallax offset", which to me suggests the calculated draw offset based on parallax, I think the property should be called something like "parallax origin" or something else that suggests that it's a reference point.

[bjorn]

I've changed the parallax origin to get subtracted, rather than added, to the view center. It seemed more intuitive to me, since I interpret the parallax origin to be a point on the map. Does that make sense?

I've also fixed the immediate updating of the layer positions when you change the property, by connecting the MapScene to the Document::changed signal and checking for this property.

The change looks fine to me now, but before we merge this we need to again update the documentation:

• Add property to index.d.ts.
• Add property to JSON and TMX format documentation, including their changelog.
• Update the https://doc.mapeditor.org/en/latest/manual/layers.html#parallax-reference-point section in the documentation. It now needs to mention the parallax origin, how it is relative to the top-left of the map's bounding box, and after that can mostly replace "map’s origin" with "parallax origin".

[krukai]

I think I covered it all. I did not test all exports yet as I am waiting for some additional feedback on whether the origin behaves as expected - I do not trust my own sanity checks alone.

[krukai]

That should be all, I think. Please have a final look.
Cheers!