vo_sixel: set --vo-sixel-fixedpalette=yes by default

fixedpalette seems to be slightly faster than dynamic
palette, and also in mlterm it avoids corruption of
too bright values overflowing to black. Hence setting
it to be default choice instead of dynamic palette.

video/out/vo_sixel.c

@@ -441,8 +441,8 @@ const struct vo_driver video_out_sixel = {
         .opt_width = 0,
         .opt_height = 0,
         .opt_reqcolors = 256,
-        .opt_fixedpal = 0,
         .opt_threshold = -1,
+        .opt_fixedpal = 1,
         .opt_top = 0,
         .opt_left = 0,
         .opt_pad_y = -1,