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

Also cv::, cv::gpu:: and cv::ocl:: namespace prefixes can be safely omitted
inside CV_ENUM and CV_FLAGS

e.g. <opencv2/core/core.hpp> become <opencv2/core.hpp>

Also renamed <opencv2/core/opengl_interop.hpp> to <opencv2/core/opengl.hpp>

Evangelidis, G.D. and Psarakis E.Z. "Parametric Image Alignment using Enhanced
Correlation Coefficient Maximization", IEEE Transactions on PAMI, vol. 32, no.
10, 2008

* added DenseOpticalFlow interface
* moved OpticalFlowDual_TVL1 to src folder

Added new performance tests for calcOpticalFlowPyrLK and buildOpticalFlowPyramid; extracted private header from lkpyramid.cpp