fftools/ffmpeg_enc: refactor setting encoding field_order

Merge three blocks with slightly inconsistent checks into one, treating
encoder input as interlaced when either:
* at least one of ilme/ildct flags is set
* the first frame in the stream is marked as interlaced
* the user specified the -top option

Stop modifying the frame passed to enc_open().

fftools/ffmpeg_enc.c

@@ -356,29 +356,18 @@ int enc_open(OutputStream *ost, AVFrame *frame)
         enc_ctx->colorspace             = frame->colorspace;
         enc_ctx->chroma_sample_location = frame->chroma_location;
 
-        // Field order: autodetection
-        if (enc_ctx->flags & (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME) &&
-            ost->top_field_first >= 0)
-            if (ost->top_field_first)
-                frame->flags |= AV_FRAME_FLAG_TOP_FIELD_FIRST;
-            else
-                frame->flags &= ~AV_FRAME_FLAG_TOP_FIELD_FIRST;
+        if (enc_ctx->flags & (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME) ||
+            (frame->flags & AV_FRAME_FLAG_INTERLACED) || ost->top_field_first >= 0) {
+            int top_field_first = ost->top_field_first >= 0 ?
+                ost->top_field_first : !!(frame->flags & AV_FRAME_FLAG_TOP_FIELD_FIRST);
 
-        if (frame->flags & AV_FRAME_FLAG_INTERLACED) {
             if (enc->id == AV_CODEC_ID_MJPEG)
-                enc_ctx->field_order = (frame->flags & AV_FRAME_FLAG_TOP_FIELD_FIRST) ? AV_FIELD_TT:AV_FIELD_BB;
+                enc_ctx->field_order = top_field_first ? AV_FIELD_TT : AV_FIELD_BB;
             else
-                enc_ctx->field_order = (frame->flags & AV_FRAME_FLAG_TOP_FIELD_FIRST) ? AV_FIELD_TB:AV_FIELD_BT;
+                enc_ctx->field_order = top_field_first ? AV_FIELD_TB : AV_FIELD_BT;
         } else
             enc_ctx->field_order = AV_FIELD_PROGRESSIVE;
 
-        // Field order: override
-        if (ost->top_field_first == 0) {
-            enc_ctx->field_order = AV_FIELD_BB;
-        } else if (ost->top_field_first == 1) {
-            enc_ctx->field_order = AV_FIELD_TT;
-        }
-
         break;
         }
     case AVMEDIA_TYPE_SUBTITLE: