prores: Set the bits_per_coded_sample for alpha pix_fmt

Improve the compatibility with other software.

libavcodec/proresenc.c

@@ -1151,6 +1151,7 @@ static av_cold int encode_init(AVCodecContext *avctx)
             av_log(avctx, AV_LOG_ERROR, "alpha bits should be 0, 8 or 16\n");
             return AVERROR(EINVAL);
         }
+        avctx->bits_per_coded_sample = 32;
     } else {
         ctx->alpha_bits = 0;
     }