This will allow to override CUDA CMake variables from first CMake call,
like cmake -DCUDA_TOOLKIT_ROOT_DIR=... -DCUDA_CUDA_LIBRARY=...

do not unset variables if CUDA_TOOLKIT_TARGET_DIR is not defined

* use opaque algorithm interfaces
* add stream support

it caused runtime failures

Improved documentation and method comment to clarify that LineSegmentDetector::detect method accepts both Vec4i and Vec4f as return vector.

Let LineSegmentDetector output line segment coordinates in float precision, as supported by the LSD algorithm.