vo_sixel: change default dither to "auto"

For two reasons:
1. It was counter intuitive that there's an "auto" value (which is
   actually a libsixel value and not an mpv one), but it's not the
   default value - our default was Atkinson.
2. "auto" provides better dithering than Atkinson with libsixel, which
   is especially noticeable with smooth gradients - where Atkinson has
   visible banding.

In libsixel 1.8.2 the "auto" value maps to Atkinson if the output
palette has up to 16 colors, or to Floyd-Steinberg otherwise (e.g.
using fixed palette with 256 colors chooses Floyd-Steinberg).

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