An easing function is a function from [0,1] to the real numbers (but usually
numbers in [0,1]) that is used to construct animation that move in a
pleasing way.
All easing functions must return 0.0 for input 0.0 and 1.0 for input 1.0.
Construct an easing function that has the first half of its output from
this easing function, and the second half from that easing function.
Construct an easing function that has the first half of its output from
this easing function, and the second half from that easing function.
For the input [0, 0.5) the resulting function uses this easing function,
and for [0.5, 1] uses that easing function. The input to the two easing
functions is linearly scaled so that they both receive a value in the
range [0, 1]. Their output is scaled in half so the first function
generates values in [0, 0.5] and the second in [0.5, 1.0].