Prevent CanvasGroup children from inheriting modulate

[romlok]

As stated in the documentation, and in the comments to godotengine/godot#54211, to modulate the children of a CanvasGroup as a single combined texture, one must use the self_modulate property.

However, this is an infeasible solution if one wishes to fade out eg. an entire game UI, at once.
The method of tweening modulate:a on the root node of the scene means that this will propagate to all children - including the children of the CanvasGroup, which will fade out individually rather than as a combined group.

The current behaviour to me seems entirely unexpected and unintuitive, since treating the CanvasGroup's children as a single unit appears to be the entire point of the class existing:

CanvasGroup¶

Merges several 2D nodes into a single draw operation.

Description¶

Child CanvasItem nodes of a CanvasGroup are drawn as a single object. It allows to e.g. draw overlapping translucent 2D nodes without blending...

So I propose that CanvasGroup should prevent modulate from cascading to its children, and instead apply a self_modulate to itself - or something equivalent to that effect.