Spotted via -Wunsequenced.
Easing equations had different behaviours depending on the toolchain
due to its abusing of inline assignments.
(cherry picked from commit 3936da7ab4)
It was Godot-ified and integrated into Tween originally (#628)
without mention that it was thirdparty code, but it's actually
derived from https://github.com/jesusgollonet/ofpennereasing.
It's also very bad quality code and should be replaced by a better,
properly-maintained library of easing equations.