use floats in snis_slider to make scaling a little cleaner
fix slider fuzzing logic
adjust fuzz amount on sliders now that fuzzing actually works right
stop fuzzing sliders when value is zero
fix off by one error in snis_randn()

Thanks to Jeremy Van Grinsven for pointing this out.

And it will be faster too because SNIS_RAND_MAX + 1 is a power of 2.