videoflip: reset orientation on new stream

Fix the following use:
- upstream sends a video with a rotation tag, say 90°
- upstream switches to another video without rotation
- the second video was still rotated by videoflip

Fix this by resetting the orientation when receiving STREAM_START.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/4377>

subprojects/gst-plugins-good/gst/videofilter/gstvideoflip.c

@@ -1790,6 +1790,10 @@ gst_video_flip_sink_event (GstBaseTransform * trans, GstEvent * event)
         gst_video_flip_set_method (vf, method, TRUE);
       }
       break;
+    case GST_EVENT_STREAM_START:
+      GST_DEBUG_OBJECT (vf, "new stream, reset orientation from tags");
+      gst_video_flip_set_method (vf, GST_VIDEO_ORIENTATION_IDENTITY, TRUE);
+      break;
     default:
       break;
   }

subprojects/gst-plugins-good/tests/check/elements/videoflip.c

@@ -369,6 +369,15 @@ GST_START_TEST (test_orientation_tag)
   // caps is updated as the frame is now rotated
   caps_update (flip, &in_info, TRUE);
 
+  // orientation is reset on STREAM_START
+  gst_harness_push_event (flip, gst_event_new_stream_start ("2"));
+
+  e = gst_harness_pull_event (flip);
+  fail_unless_equals_int (GST_EVENT_TYPE (e), GST_EVENT_STREAM_START);
+  gst_event_unref (e);
+
+  caps_update (flip, &in_info, FALSE);
+
   gst_harness_teardown (flip);
 }