Change Node*D::reparent to set its new transform before reparenting

[mihe]

Fixes #113058.

The current implementation of Node*D::reparent, when called with keep_global_transform at its default of true, does the following:

1. Cache the previous global transform.
2. Remove the node from its current parent, thus emitting NOTIFICATION_EXIT_WORLD.
3. Add the node to its new parent, thus emitting NOTIFICATION_ENTER_WORLD, but with a potentially new (incorrect) global transform, since the local transform is what's actually preserved in the transition.
4. Set the previous global transform again (and thus a new local transform).

This creates problems for nodes where the value of the global transform is important at the time of NOTIFICATION_ENTER_WORLD, like for example kinematic physics bodies, such as CharacterBody3D¹.

This PR "fixes" that by changing Node*D::reparent to instead set the new local transform before we do remove_child/add_child, so that the node already has the correct global transform by the time add_child is called.

This of course means that now we won't have the correct local/global transform when NOTIFICATION_EXIT_WORLD/NOTIFICATION_EXIT_TREE is emitted instead, so arguably just trades one bug for another, but I figured this might prompt some discussion for better a solution.

The proper solution to this (at least in my mind) would be to instead restore the previous global transform inbetween these two lines, as opposed to before/after them:

godot/scene/main/node.cpp

Lines 2077 to 2078 in 7ed0b61

	data.parent->remove_child(this);
	p_parent->add_child(this);

... but I struggle to come up with a clean way of doing so.

Footnotes

1. This seems to have "worked" with Godot Physics due to what to me looks like a bug in that implementation, but which is not present in the Jolt Physics implementation. ↩

[fire]

See #74659 for an alternative solution for get_global_transform()