added gpu BM optical flow implementation

modules/gpu/include/opencv2/gpu/gpu.hpp

@@ -2074,6 +2074,24 @@ private:
 };
 
 
+//! Calculates optical flow for 2 images using block matching algorithm */
+CV_EXPORTS void calcOpticalFlowBM(const GpuMat& prev, const GpuMat& curr,
+                                  Size block_size, Size shift_size, Size max_range, bool use_previous,
+                                  GpuMat& velx, GpuMat& vely, GpuMat& buf,
+                                  Stream& stream = Stream::Null());
+
+class CV_EXPORTS FastOpticalFlowBM
+{
+public:
+    void operator ()(const GpuMat& I0, const GpuMat& I1, GpuMat& flowx, GpuMat& flowy, int search_window = 21, int block_window = 7, Stream& s = Stream::Null());
+
+private:
+    GpuMat buffer;
+    GpuMat extended_I0;
+    GpuMat extended_I1;
+};
+
+
 //! Interpolate frames (images) using provided optical flow (displacement field).
 //! frame0   - frame 0 (32-bit floating point images, single channel)
 //! frame1   - frame 1 (the same type and size)