YCbCr output support

Motivated by vf_libplacebo.

Needs testing.

src/include/libplacebo/utils/libav_internal.h

@@ -472,6 +472,7 @@ static inline void pl_frame_from_avframe(struct pl_frame *out,
     } else if (desc->flags & AV_PIX_FMT_FLAG_RGB) {
 
         out->repr.sys = PL_COLOR_SYSTEM_RGB;
+        out->repr.levels = PL_COLOR_LEVELS_FULL; // libav* ignores levels for RGB
 
     } else if (!out->repr.sys) {