BezierEase

Imagine a 2D bezier curve from 0,0 to 1,1 using the intermediate points x1,y1 and x2,y2.

The x value is equivalent to the t value in ease, and the y value is the result of ease.

In order the work out the y value, we first need to calculated how "far" along (in time) the bezier curve we are. Annoyingly this is also called "t", but is unrelated to the parameter of the ease function.