Support negative numbers in cubic-bezier(...) function

[colelawrence]

Required to support ease in back / bounce kinds of easings.

Tested with mutation testing alongside cargo test test-driver-interpreter.

[colelawrence]

This is just for the interpreter, so there might be a better place to put this.

[colelawrence]

Question if this code over here is relevant @ogoffart for this change, or if everything is going to be fine. I can observe that overshooting works on animations. I just might ben having trouble understanding what this code affects.

slint/internal/core/animations.rs

Lines 281 to 287 in eb1941b

	pub fn easing_curve(curve: &EasingCurve, value: f32) -> f32 {
	match curve {
	EasingCurve::Linear => value,
	EasingCurve::CubicBezier([a, b, c, d]) => {
	if !(0.0..=1.0).contains(a) && !(0.0..=1.0).contains(c) {
	return value;
	};

[ogoffart]

Question if this code over here is relevant

In CSS, the first and third number need to be between 0 and 1, but the second an last can be anything.

[ogoffart]

Thanks for the patch!