feat: Implement Optical Flow KPS tracking for webcam performance

Introduces Nth-frame full face detection combined with KCF bounding
box tracking and Lucas-Kanade (LK) optical flow for keypoint (KPS)
tracking on intermediate frames. This is primarily for single-face
webcam mode to improve performance while maintaining per-frame swaps.

Key Changes:
- Modified `face_swapper.py` (`process_frame`):
    - Full `insightface.FaceAnalysis` runs every N frames (default 5)
      or if tracking is lost.
    - KCF tracker updates bounding box on intermediate frames.
    - Optical flow (`cv2.calcOpticalFlowPyrLK`) tracks the 5 keypoints
      from the previous frame to the current intermediate frame.
    - A `Face` object is constructed with tracked bbox and KPS for
      swapping on intermediate frames (detailed landmarks like
      `landmark_2d_106` are None for these).
    - Experimental similar logic added to `_process_live_target_v2`
      for `map_faces=True` live mode (non-many_faces path).
- Robustness:
    - Mouth masking and face mask creation functions in `face_swapper.py`
      now handle cases where `landmark_2d_106` is `None` (e.g., by
      skipping mouth mask or using bbox for face mask).
    - Added division-by-zero check in `apply_color_transfer`.
- State Management:
    - Introduced `reset_tracker_state()` in `face_swapper.py` to clear
      all tracking-related global variables.
    - `ui.py` now calls `reset_tracker_state()` at appropriate points
      (webcam start, mode changes, new source image selection) to ensure
      clean tracking for new sessions.
- `DETECTION_INTERVAL` in `face_swapper.py` increased to 5.

This aims to provide you with a smoother face swap experience with better FPS
by reducing the frequency of expensive full face analysis, while the
actual swap operation continues on every frame using tracked data.

modules/ui.py

@@ -19,6 +19,7 @@ from modules.face_analyser import (
 )
 from modules.capturer import get_video_frame, get_video_frame_total
 from modules.processors.frame.core import get_frame_processors_modules
+from modules.processors.frame.face_swapper import reset_tracker_state # Added import
 from modules.utilities import (
     is_image,
     is_video,
@@ -240,6 +241,7 @@ def create_root(start: Callable[[], None], destroy: Callable[[], None]) -> ctk.C
         command=lambda: (
             setattr(modules.globals, "many_faces", many_faces_value.get()),
             save_switch_states(),
+            reset_tracker_state() # Added reset call
         ),
     )
     many_faces_switch.place(relx=0.6, rely=0.65)
@@ -266,7 +268,8 @@ def create_root(start: Callable[[], None], destroy: Callable[[], None]) -> ctk.C
         command=lambda: (
             setattr(modules.globals, "map_faces", map_faces.get()),
             save_switch_states(),
-            close_mapper_window() if not map_faces.get() else None
+            close_mapper_window() if not map_faces.get() else None,
+            reset_tracker_state() # Added reset call
         ),
     )
     map_faces_switch.place(relx=0.1, rely=0.75)
@@ -604,9 +607,11 @@ def select_source_path() -> None:
         RECENT_DIRECTORY_SOURCE = os.path.dirname(modules.globals.source_path)
         image = render_image_preview(modules.globals.source_path, (200, 200))
         source_label.configure(image=image)
+        reset_tracker_state() # Added reset call
     else:
         modules.globals.source_path = None
         source_label.configure(image=None)
+        reset_tracker_state() # Added reset call even if source is cleared
 
 
 def swap_faces_paths() -> None:
@@ -979,6 +984,8 @@ def create_webcam_preview(camera_index: int):
     frame_count = 0
     fps = 0
 
+    reset_tracker_state() # Ensure tracker is reset before starting webcam loop
+
     while True:
         ret, frame = cap.read()
         if not ret: