prores: do not set pixel format on codec init

The pixel format is not known until the frame header is parsed.
Guessing it here only causes trouble for the caller if the guess
turns out to be wrong (and actually causes very wrong output by
avconv/avplay).
Signed-off-by: Mans Rullgard <mans@mansr.com>

libavcodec/proresdec.c

@@ -105,8 +105,6 @@ static av_cold int decode_init(AVCodecContext *avctx)
     ctx->total_slices     = 0;
     ctx->slice_data       = NULL;
 
-    avctx->pix_fmt = PIX_FMT_YUV422P10; // set default pixel format
-
     avctx->bits_per_raw_sample = PRORES_BITS_PER_SAMPLE;
     ff_proresdsp_init(&ctx->dsp);
 

tests/ref/fate/prores-alpha

-0, 0, 12441600, 0x5188265d
+0, 0, 12441600, 0x79c18863
 0, 3003, 12441600, 0x79c18863