libavcodec/xsubenc.c: set coded_bits_per_sample

Explicitly set the number of bits per sample to stop
the avi muxer from guessing at 24 bits.
The result is that bits per pixel in the avi stream header
matches what other XSUB muxers produce, such as AVIAddXSubs.exe

Signed-off-by: Erik Olofsson <eaj.olofsson@gmail.com>
Reviewed-by: Reimar Döffinger <Reimar.Doeffinger@gmx.de>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/xsubenc.c

@@ -206,6 +206,8 @@ static av_cold int xsub_encoder_init(AVCodecContext *avctx)
     if (!avctx->codec_tag)
         avctx->codec_tag = MKTAG('D','X','S','B');
 
+    avctx->bits_per_coded_sample = 4;
+
     return 0;
 }