Bug 922770 - Set stable frame threshold to be less than 4096 pixels;r=davehunt

Turns out that 2048 was too few for certain cases (e.g. a scrollbar
transitioning out) where things aren't actually changing very much.

src/videocapture/videocapture/framediff.py

@@ -89,12 +89,12 @@ def get_num_unique_frames(capture, threshold=0):
 def get_fps(capture, threshold=0):
     return get_num_unique_frames(capture, threshold=threshold) / capture.length
 
-def get_stable_frame(capture, threshold = 2048):
+def get_stable_frame(capture, threshold = 4096):
     framediff_sums = get_framediff_sums(capture)
     for i in range(len(framediff_sums)-1, 0, -1):
         if framediff_sums[i] > threshold:
             return i+1
     return len(framediff_sums)-1
 
-def get_stable_frame_time(capture, threshold = 2048):
+def get_stable_frame_time(capture, threshold = 4096):
     return get_stable_frame(capture, threshold) / 60.0