allow changing AnimatedValue properties without losing current progress

This also means changing easing function will not modify the progress

src/itdelatrisu/opsu/ui/animations/AnimatedValue.java

@@ -58,6 +58,21 @@ public class AnimatedValue {
 		this.eqn = eqn;
 	}
 	
+	public void change(int duration, float min, float max, AnimationEquation eqn) {
+		float progress = (float) this.time / this.duration;
+		if (this.eqn != eqn) {
+			if (this.time != 0 && this.time != this.duration) {
+				progress = this.eqn.uncalc(progress);
+			}
+			this.eqn = eqn;
+		}
+		this.duration = duration;
+		this.time = (int) (this.duration * progress);
+		this.base = min;
+		this.diff = max - min;
+		this.updateValue();
+	}
+	
 	/**
 	 * Change the from and to values
 	 * @param min start value

src/itdelatrisu/opsu/ui/animations/AnimationEquation.java

@@ -312,4 +312,19 @@ public enum AnimationEquation {
 	 * @return the new {@code t} value [0,1]
 	 */
 	public abstract float calc(float t);
+	
+	public float uncalc(float x) {
+		float min = 0f;
+		float max = 1f;
+		// 7 iterations is already decent, do we need 8?
+		for (int i = 0; i < 8; i++) {
+			float pos = (min + max) / 2f;
+			if (this.calc(pos) > x) {
+				max = pos;
+			} else {
+				min = pos;
+			}
+		}
+		return (min + max) / 2f;
+	}
 }