Fix `Animation::subtract_variant` for affine transforms

[kleonc]

Transform2D::inverse/Transform3D::inverse which were used in Animation::subtract_variant work only for rotation + translation, don't work for scale, skew, etc.

This resulted in e.g. PropertyTweener incorrectly calculating the delta for affine transforms: https://github.com/godotengine/godot/blob/b8ed596769d2114797015833ce86f86ee872ecfa/scene/animation/tween.cpp#L557

Fixes #79242.