Support switching field order when decoding frwu.

The binary encoder has a "switch field order" setting that
moves one line from the top to the bottom of the frame.

Fixes ticket #966.

libavcodec/frwu.c

@@ -22,6 +22,12 @@
 
 #include "avcodec.h"
 #include "bytestream.h"
+#include "libavutil/opt.h"
+
+typedef struct {
+    AVClass *av_class;
+    int change_field_order;
+} FRWUContext;
 
 static av_cold int decode_init(AVCodecContext *avctx)
 {
@@ -41,6 +47,7 @@ static av_cold int decode_init(AVCodecContext *avctx)
 static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
                         AVPacket *avpkt)
 {
+    FRWUContext *s = avctx->priv_data;
     int field, ret;
     AVFrame *pic = avctx->coded_frame;
     const uint8_t *buf = avpkt->data;
@@ -84,9 +91,14 @@ static int decode_frame(AVCodecContext *avctx, void *data, int *data_size,
             av_log(avctx, AV_LOG_ERROR, "Packet is too small, need %i, have %i\n", field_size, (int)(buf_end - buf));
             return AVERROR_INVALIDDATA;
         }
-        if (field)
+        if (field ^ s->change_field_order) {
             dst += pic->linesize[0];
+        } else if (s->change_field_order) {
+            dst += 2 * pic->linesize[0];
+        }
         for (i = 0; i < field_h; i++) {
+            if (s->change_field_order && field && i == field_h - 1)
+                dst = pic->data[0];
             memcpy(dst, buf, avctx->width * 2);
             buf += avctx->width * 2;
             dst += pic->linesize[0] << 1;
@@ -110,13 +122,28 @@ static av_cold int decode_close(AVCodecContext *avctx)
     return 0;
 }
 
+static const AVOption frwu_options[] = {
+    {"change_field_order", "Change field order", offsetof(FRWUContext, change_field_order), FF_OPT_TYPE_INT,
+     {.i64 = 0}, 0, 1, AV_OPT_FLAG_DECODING_PARAM | AV_OPT_FLAG_VIDEO_PARAM},
+    {NULL}
+};
+
+static const AVClass frwu_class = {
+    .class_name = "frwu Decoder",
+    .item_name  = av_default_item_name,
+    .option     = frwu_options,
+    .version    = LIBAVUTIL_VERSION_INT,
+};
+
 AVCodec ff_frwu_decoder = {
     .name           = "frwu",
     .type           = AVMEDIA_TYPE_VIDEO,
     .id             = AV_CODEC_ID_FRWU,
+    .priv_data_size = sizeof(FRWUContext),
     .init           = decode_init,
     .close          = decode_close,
     .decode         = decode_frame,
     .capabilities   = CODEC_CAP_DR1,
     .long_name      = NULL_IF_CONFIG_SMALL("Forward Uncompressed"),
+    .priv_class     = &frwu_class,
 };