fix timestamps for videos

Author: tobybreckon

Diff for: camera_stream.py

@@ -158,7 +158,9 @@ def __init__(self, src=None, backend=None,
         # if a source was specified at init, proceed to open device
         if not (src is None):
             self.open(src, backend)
-
+            if not (backend == cv2.CAP_V4L):
+                self.use_timestamps = False
+                
     def open(self, src=0, backend=None):
 
         # determine backend to specified by user