handle case where video is so black we fail to get a thumbnail

pa_job_manager.py

@@ -1418,9 +1418,19 @@ def GenVideoThumbnail(job, file):
     try:
         vcap = cv2.VideoCapture(file)
         res, frame = vcap.read()
+        first_res = res
+        first_frame = frame
+        frame_cnt=0
         # if the mean pixel value is > 15, we have something worth making a sshot of (no black frame at start being the sshot)
-        while frame.mean() < 15 and res:
+        # limit it to 1000 frames jic
+        while res and frame.mean() < 15 and frame_cnt < 1000:
             res, frame = vcap.read()
+            frame_cnt += 1
+        # if we never got a frame that is not dark, then use first frame
+        if not res:
+            res = first_res
+            frame = first_frame
+            
         w = vcap.get(cv2.cv2.CAP_PROP_FRAME_WIDTH)
         h = vcap.get(cv2.cv2.CAP_PROP_FRAME_HEIGHT)
         if w > h: