WaterSystems instantiated into other scenes do not work correctly

[Arnklit]

Describe the bug
If a WaterSystem is instantiated into another scene, it might not be at the same coordinates and therefore the coordinates used for the height and flow maps are no longer valid. This will also cause issues with the auto assign of the textures and coordinates for use in Custom shaders as this happens on texture generation and therefore will only work withing the local scene.

Expected behavior
Have the node calculate an offset to the coordinates based on it's world position at run time. Also have the auto assign happen on enter_tree / ready as well as on generate.