[bugfix] Fix progress bar accuracy for VFR videos (#562)

- Addresses progress bar behavior mentioned in Issue #561 where progress
bar only goes up to ~80% with VFR videos, even with PyAV backend.

- Progress bar now goes up to 100% for both PyAV and OpenCV backends.

- Please note that this PR does not address the main topic of Issue #561
(Invalid end time in scene lists for VFR videos) which was determined to
be unique to the OpenCV backend and is not present when using PyAV.

Author: Breakthrough

scenedetect/scene_manager.py

@@ -571,6 +571,7 @@ def detect_scenes(
         )
         decode_thread.start()
         frame_im = None
+        prev_position = None
 
         logger.info("Detecting scenes...")
         try:
@@ -587,7 +588,13 @@ def detect_scenes(
                         progress_bar.set_description(
                             PROGRESS_BAR_DESCRIPTION % len(self._cutting_list), refresh=False
                         )
-                    progress_bar.update(1 + frame_skip)
+                    # Increment progress bar by delta of position.frame_num instead of 1
+                    # to handle VFR video where frame count is an approximation.
+                    delta = (
+                        1 if prev_position is None else position.frame_num - prev_position.frame_num
+                    )
+                    progress_bar.update(delta)
+                    prev_position = position
         finally:
             if progress_bar is not None:
                 progress_bar.set_description(