output = (2.0^input)
This is useful for converting a pitch modulation value into a frequency
scaler. An input value of +1.0 will output 2.0 for an octave increase. An
input value of -1.0 will output 0.5 for an octave decrease.
This implementation uses a table lookup to optimize for speed. It is accurate
enough for tuning.
It also checks to see if the current input value is the same as the previous
input value. If so then it reuses the previous computed value.