Feature allows to exclude CUDA or OpenCL optimizations at all even CUDA is used
on build. Exclusion of CUDA or OpenCL cut unwanted dependencies.

2. Let Farneback to be the default optical flow method.
3. Fix a timing method bug for ocl path.
4. Remove useless finish operation in farneback
5. Refactor buffer usage.

Looks like we don't actually use them; when creating precompiled headers
with Visual C++, a dummy .cpp is created at build time.

Accuracy and performance tests are also provided.

Backport from master.