prores_raw: parse the frame before honouring skip_frame

The decoder returned before parsing anything when skip_frame was
AVDISCARD_ALL, yet advertises FF_CODEC_CAP_SKIP_FRAME_FILL_PARAM, so it
never set the pixel format and dimensions while probing. Probing gives up
after probesize, and ProRes RAW frames are larger than the default, so
ffmpeg failed to open such files at all with "Could not find codec
parameters ... unspecified pixel format". Parse up to the tile table, which
sets every stream parameter, before skipping the frame.

libavcodec/prores_raw.c

@@ -337,9 +337,6 @@ static int decode_frame(AVCodecContext *avctx,
     DECLARE_ALIGNED(32, uint8_t, qmat)[64];
     memset(qmat, 1, 64);
 
-    if (avctx->skip_frame >= AVDISCARD_ALL)
-        return avpkt->size;
-
     switch (avctx->codec_tag) {
     case 0:
         break;
@@ -517,6 +514,11 @@ static int decode_frame(AVCodecContext *avctx,
     }
     av_assert1(n == s->nb_tiles);
 
+    /* The stream parameters are all set by now, which is all that probing
+     * with frames skipped needs */
+    if (avctx->skip_frame >= AVDISCARD_ALL)
+        return avpkt->size;
+
     /**
      * Any data between last tile and frame end is vendor-specific metadata:
      * [psim record]  4 be32 size + "psim" + pascal string + payload