gpu TVL1 Optical Flow optimization:

do not calculate sum of error in every round of iteration;
instead the error will be summed every 2nd times or more, 
if the previous sum of error is too far away from threshold.

modules/gpu/perf/perf_video.cpp

@@ -427,8 +427,8 @@ PERF_TEST_P(ImagePair, Video_OpticalFlowDual_TVL1,
 
         TEST_CYCLE() d_alg(d_frame0, d_frame1, u, v);
 
-        GPU_SANITY_CHECK(u, 1e-2);
-        GPU_SANITY_CHECK(v, 1e-2);
+        GPU_SANITY_CHECK(u, 1e-1);
+        GPU_SANITY_CHECK(v, 1e-1);
     }
     else
     {