VideoCapture: copy the captured frame, to avoid dangling Mats

Previously, VideoCapture::retrieve would return a Mat that referenced
the internal IplImage. Since the latter is rewritten every time a
frame is captured, it means that if the user captures two frames in a row,
the first frame would reference nothing. Similar if a user captures a frame,
then destroys the VideoCapture instance.

Note that the other branch of the if isn't affected, since flip allocates
a new Mat.
fix crash when path has spaces
The paths are defined properly with an escape "\ " but you cannot have an escape and
quotes when piping (otherwise, escapes are understood as 2 characters).
So just remove the quotes.
Overload detectMultiScale API for ocl::haar.
fixed ocl::cornerHarris, ocl::cornerMinEigenVal and their accuracy tests
fixed warnings in OpenCL kernels
Let perf/accuracy test of ocl haar uses detectMultiScale api.
Fix image to be used by perf test.
added ocl::repeat
came back to relative error
fix the crash as suggested by @SpecLad
add VERBATIM as advised by @SpecLad
ocl: update documentation
ocl: update comments in ocl.hpp
Revert back test image.
Merge pull request #1743 from ilya-lavrenov:ocl_repeat
Merge pull request #1739 from pengx17:2.4_ocl_overload_haar
Merge pull request #1740 from ilya-lavrenov:ocl_corners
Merge pull request #1750 from alalek:ocl_update_documentation
ocl: workaround for subbuffer memory leaks
Android Manager Version++.
Merge pull request #1711 from SpecLad:cap-broken-mat