Fixes ticket #7441.

for block contrast calculate, the block is like this:

|<---------------- stride-----------------------|
+----------------------------------------------->  X
|
|            w = 16
|    (cx,cy)+------+
|           |      |
|h=blocksize|      |
|           |      |
|           +------+
V

Y

so we calc the block contrast use:
   (cy + y) * stride + (cx + x)
Signed-off-by: Jun Zhao <mypopydev@gmail.com>