updated gpu module API

This PR breaks binary compatibility for gpu module:

* changed public members of CannyBuf struct due to new implementation
* renamed BruteForceMatcher_GPU_base to BFMatcher_GPU (similar to cpu analogue)
* removed some members from PyrLKOpticalFlow due to new implementation
* removed some members from FarnebackOpticalFlow due to new implementation
* made some methods of SURF_GPU, FAST_GPU, ORB_GPU static