clamping of noise octaves in Turbulence

[bugmenot2]

Omitting octaves essentially replaces them by the constant 0. This is only correct if the average of an octave is zero, as is the case for the Noise() function used in FBm.

But the average of the fabsf(Noise()) function used in Turbulence is non-zero. It is in fact the standard deviation of the Noise() function (roughly 0.2). Omitted octaves should be replaced by that non-zero constant or the result will be too small. In fact, if the maximum differential dpdx or dpdy > 2 (e.g. at grazing angles), the number of octaves n < 0, and the result will be black instead of gray.

[mmp]

Nice one, thanks. Fix just pushed to github.