vo_gpu: lower default deband threshold

The previous default was found to be too aggressive for most video.
Change to a lower value to prevent destroying too much detail.

video/out/gpu/video_shaders.c

@@ -946,7 +946,7 @@ struct deband_opts {
 
 const struct deband_opts deband_opts_def = {
     .iterations = 1,
-    .threshold = 64.0,
+    .threshold = 32.0,
     .range = 16.0,
     .grain = 48.0,
 };