Noise Changes

### 2.6.1 Noise Changes

The noise function used in POV-Ray 3.1 results in "plateaus" which sometimes leads to unwanted results. Therefore two alternative noise generators have been introduced, controlled with the `noise_generator` keyword. Adding

      noise_generator TYPE

to `global_settings{}` specifies the desired noise function globally, or you can use the same keyword after any pattern to select a noise generator just for a single case. The default generator is type 2

You can choose `noise_generator 1`, which behaves like the noise in POV-Ray 3.1 and earlier, or `noise_generator 2`, which is a version of type 1 squeezed down into a smaller range to prevent the plateaus. The generator type 3 is a completely new Perlin noise function.