optflow: RLOF fixes (python-binding, status flag, flat textures) (#2097)

* * Bugfix for python-binding related to issue #2094. Copying the prevPtsMat to prevPoints was broken, when using python-binding.

* * Connect the blurred image correctly to CROSS support region segmentation. This allows to compute more robust support region shapes. Commit refers to issue #2094.

* * Bugfix to avoid the unintended assertion on flat textures discussed in issue #2094.

* * Bugfix on SparseRLOFOpticalFlow status was wrongly to to 0 if forward backward error < threshold, which is wrong. Inequation has been corrected. Refers to in issue #2087.

* small documentation changes

* bugfix on assertions and small test changes