Fix get_parent_anchorable_rect() not returning the correct size in some cases

[MinusKube]

Fixes #67303

When a control is at the top-level of a subviewport (e.g. in a Window or in a SubviewportContainer), it was considered as if it was at the top-level of the edited scene viewport, thus the parent anchorable rect was computed using the viewport_width and viewport_height properties, instead of using the real subviewport size.

[akien-mga]

CC @Sauermann

[Sauermann]

I have tested the PR and it fixes the MRP of the linked issue.

However a special case is not taken into consideration:

1. Take the MRP of the linked issue
2. In the SceneTree make Window to scene-root

These steps result in the following SceneTree:

• Window
  • TextureRect

In this case the TextureRect is again not displayed correctly.

[MinusKube]

Okay, fixed! Now I compare the current viewport to the edited scene root's parent viewport instead of the root viewport directly.

[YuriSizov]

Thanks!