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.
SimplexNoise can be used to generate parameterized fractal noise based on Open Simplex.
NoiseTexture uses SimplexNoise to generate noise textures for using in
shaders/visual effects.
This allows more consistency in the manner we include core headers,
where previously there would be a mix of absolute, relative and
include path-dependent includes.