ffmpeg: fix order between field order autodetection and override

Having the override before autodetection meant that the overridden
value got overwritten by the autodetected result each time,
effectively disabling the ability to utilize the `-top` option
for override purposes.

Somehow I missed this in fbb44bc ,
even though the lines were within the context. Probably the code
originally being after this logic had something to do with it,
but previously it only touched the avformat context's codecpar,
which did not affect the encoder codec context whatsoever.

Fixes #9320
Fixes #9339

fftools/ffmpeg.c

@@ -3483,12 +3483,7 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
             enc_ctx->bits_per_raw_sample = frame_bits_per_raw_sample;
         }
 
-        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;
-        }
-
+        // Field order: autodetection
         if (frame) {
             if (enc_ctx->flags & (AV_CODEC_FLAG_INTERLACED_DCT | AV_CODEC_FLAG_INTERLACED_ME) &&
                 ost->top_field_first >= 0)
@@ -3503,6 +3498,13 @@ static int init_output_stream_encode(OutputStream *ost, AVFrame *frame)
                 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;
+        }
+
         if (ost->forced_keyframes) {
             if (!strncmp(ost->forced_keyframes, "expr:", 5)) {
                 ret = av_expr_parse(&ost->forced_keyframes_pexpr, ost->forced_keyframes+5,