GitHub#465 Fix NonMaxSuppression

GitHub#465 Fix NonMaxSuppression

If visited flag set true, it should not be visited any more.
For example, if we put 10 boxes (ordered by score) into NonMaxSuppression:
* Step1: Suppose Box 2/3/6/8 are suppressed by Box 1. Box 4/5/7/9/10 survived.
* Step2: Correct way: We use Box 4 to suppress the survive boxes.
         Prior to this commit: Box 4 may be suppressed by Box 2,
          even Box 2 is already suppressed by Box 1...

Signed-off-by: Antkillerfarm <antkillerfarm@gmail.com>
Change-Id: I38d7a84287649827a16565748592fb562b4df5d5