A cubic Bézier easing function
The function is defined by two points, (x1, y1), and (x2, y2) that define the P₁ and P₂ control points of a cubic Bézier curve. The curve's endpoints P₀ and P₃ are fixed at (0, 0) and (1, 1) respectively.
x1 and x2 must be in the range [0, 1].