Improve object tracking (#17671)

* Save initial frame of new objects to frame cache
Objects that move quickly through the frame and are only seen briefly may not have the update() method called to save thumbnail_data, and may not have the initial frame saved to the tracked object frame cache. This caused a "Frame missing from frame cache" message that was patched by #7313 but this sometimes caused the wrong frame to be chosen for the thumb/snapshot.

* Tracking tweaks
- When registering new objects, use the past detections from Norfair to populate self.positions and self.stationary_box_history. This prevents the first call of update_position() from triggering a +1 on the object's stationary count (because the iou would be 1.0).
- Add a specific tracker for dedicated LPR cam license_plate objects using a lower R value and higher distance threshold to account for fast moving plates.
- Add helpful debug messages and keep them disabled with `if False:`

frigate/camera/state.py

@@ -263,6 +263,27 @@ class CameraState:
                 current_detections[id],
             )
 
+            # add initial frame to frame cache
+            self.frame_cache[frame_time] = np.copy(current_frame)
+
+            # save initial thumbnail data and best object
+            thumbnail_data = {
+                "frame_time": frame_time,
+                "box": new_obj.obj_data["box"],
+                "area": new_obj.obj_data["area"],
+                "region": new_obj.obj_data["region"],
+                "score": new_obj.obj_data["score"],
+                "attributes": new_obj.obj_data["attributes"],
+                "current_estimated_speed": 0,
+                "velocity_angle": 0,
+                "path_data": [],
+                "recognized_license_plate": None,
+                "recognized_license_plate_score": None,
+            }
+            new_obj.thumbnail_data = thumbnail_data
+            tracked_objects[id].thumbnail_data = thumbnail_data
+            self.best_objects[new_obj.obj_data["label"]] = new_obj
+
             # call event handlers
             for c in self.callbacks["start"]:
                 c(self.name, new_obj, frame_name)