Importer does not implement bgPos property on LDTKLevels

[the-simian]

currently ldtk supports background modes:

enum BackgroundMode {
	UNSCALED,
	COVER_DIRTY,
	COVER,
	REPEAT,
	FIT_INSIDE
}

In addition to the __bgPos which can derive the anchor point, we also need to adapt the background modes at imports.

I've got this working and will support this issue with a PR

[the-simian]

resolved by https://github.com/heygleeson/godot-ldtk-importer/pull/51