imgconvert: make avcodec_find_best_pix_fmt2() more bug robust against future changes

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/imgconvert.c

@@ -488,6 +488,7 @@ enum PixelFormat avcodec_find_best_pix_fmt2(enum PixelFormat dst_pix_fmt1, enum
         ~FF_LOSS_DEPTH,
         ~(FF_LOSS_RESOLUTION | FF_LOSS_DEPTH | FF_LOSS_COLORSPACE | FF_LOSS_ALPHA |
           FF_LOSS_COLORQUANT | FF_LOSS_CHROMA),
+        0x80000, //non zero entry that combines all loss variants including future additions
         0,
     };