Define snow .pix_fmts to allow colour space auto-selection.

libavcodec/snowenc.c

@@ -1938,6 +1938,10 @@ AVCodec ff_snow_encoder = {
     .init           = encode_init,
     .encode2        = encode_frame,
     .close          = encode_end,
+    .pix_fmts       = (const enum PixelFormat[]){
+        PIX_FMT_YUV420P, PIX_FMT_YUV410P, PIX_FMT_YUV444P,
+        PIX_FMT_NONE
+    },
     .long_name      = NULL_IF_CONFIG_SMALL("Snow"),
     .priv_class     = &snowenc_class,
 };