dither: don't use long double

This fixes compilation with broken libcs, like on Cygwin. C99
absolutely requires long double and associated functions like expl,
even if long double is double. But newlib (used by cygwin) omits
declaration for these if long double is equivalent to double.

The extra precision is not needed here, so remove it to make life
easier for the single person using mpv with cygwin.