[Zig] fix compile error, domain warp amp usage

[wilba]

1. Fix compile error, use of const instead of var for variables that are modified.
2. Fix applying fractal bounding to domain warp amplitude when the fractal type is .none. Note this matches the behavior if either octaves is zero or gain is zero, which would calculate fractal bounding of 1.0 and thus not change domain warp amplitude.
3. Fix calculateFractalBounding(), loop starts at 1 in the C implementation.

Tagging @ForeverZer0 who did the Zig port in case they are interested.