Remove useless exponentiation in fbm by multiplying the value in the loop

[bertin0]

Fixes #299

[PrinceOfBorgo]

The fix is wrong. Multiplying attenuation by itself at each iteration produces persistence ^ (2 ^ (x + 1)) and not persistence ^ (x + 1). The fix is to multiply attenuation by self.persistance instead.

// Increase the attenuation for the next octave, to be equal to persistence ^ (x + 1)
attenuation *= self.persistence;