Popup window aspect artifacts

[acgc99]

This is an scaling artifact caused by display/window/stretch/aspect = expand, with ignore/keep there is no problem. Related to #18. This can be solved with anchors.

[acgc99]

This can be solved with anchors I hope, like #18. I decided to use appearing/disappearing animations to bypass the problem, I am not very fluent with anchors and I could not be able to get a full solution.

[acgc99]

I recovered shrink/expand behavior at 20eab48. You have to anchors preset to full rect and prepare position and scale manually at the scene where the popup is placed:

func _ready() -> void:
	popup.position = size/2.0
	popup.scale = Vector2.ZERO

Here size corresponds to the viewport size (because parent node has an anchors preset of full rect), that size might be different from the one specified on the project settings depending on Project Settings > Display > Windows > Stretch options.