avcodec/h264_slice: set the SEI parameters early on the AVCodecContext

It's better to do it before the buffers are actually created. At least in VLC
we currently don't support changing some parameters dynamically easily so we
don't use the information if it comes after the buffer are created.
Co-authored-by: James Almer <jamrial@gmail.com>
Signed-off-by: James Almer <jamrial@gmail.com>

libavcodec/h264_slice.c

@@ -1092,6 +1092,12 @@ static int h264_init_ps(H264Context *h, const H264SliceContext *sl, int first_sl
                 h->avctx->colorspace      = sps->colorspace;
             }
         }
+
+        if (h->sei.alternative_transfer.present &&
+            av_color_transfer_name(h->sei.alternative_transfer.preferred_transfer_characteristics) &&
+            h->sei.alternative_transfer.preferred_transfer_characteristics != AVCOL_TRC_UNSPECIFIED) {
+            h->avctx->color_trc = h->sei.alternative_transfer.preferred_transfer_characteristics;
+        }
     }
 
     if (!h->context_initialized || must_reinit || needs_reinit) {
@@ -1332,12 +1338,6 @@ static int h264_export_frame_props(H264Context *h)
         h->sei.picture_timing.timecode_cnt = 0;
     }
 
-    if (h->sei.alternative_transfer.present &&
-        av_color_transfer_name(h->sei.alternative_transfer.preferred_transfer_characteristics) &&
-        h->sei.alternative_transfer.preferred_transfer_characteristics != AVCOL_TRC_UNSPECIFIED) {
-        h->avctx->color_trc = cur->f->color_trc = h->sei.alternative_transfer.preferred_transfer_characteristics;
-    }
-
     return 0;
 }