updated video_threaded.py sample:

- switch btw threaded and non-threaded mode
- performance counters
- description

samples/python2/common.py

@@ -126,6 +126,17 @@ def Timer(msg):
     finally:
         print "%.2f ms" % ((clock()-start)*1000)
 
+class StatValue:
+    def __init__(self, smooth_coef = 0.5):
+        self.value = None
+        self.smooth_coef = smooth_coef
+    def update(self, v):
+        if self.value is None:
+            self.value = v
+        else:
+            c = self.smooth_coef
+            self.value = c * self.value + (1.0-c) * v
+
 class RectSelector:
     def __init__(self, win, callback):
         self.win = win