Parse canvasConfig width and height if present

[Peregrine05]

Chunky has a canvasConfig now, so this PR updates the scene parser in the scene loader dialog to use the canvasConfig width and height values, if present, and to fall back to the scene JSON root width and height otherwise.

[leMaik]

The scene JSON does not have a canvasConfig object. The Java CanvasConfig object flattens its configuration into the root scene object.

[Peregrine05]

Thanks for catching that. When I saw the issue present, I thought it would also be present on master (like with the resin bricks issue) but I forgot to actually check.

[leMaik]

tbh I was confused for a moment too and had to check the code on master first 😅