Add OpenCL  support to linearPolar & logPolar.
The OpenCL code use float instead of double, so that it does not require
cl_khr_fp64 extension, with slight precision lost.

Add explicit conversion

Add explicit conversion from double to float to eliminate warning during
compilation.

https://github.com/Itseez/opencv/issues/6456

image.forEachWithPosition does not exist

templated std::vector<T> constructor was being called instead of const std::vector<cv::cuda::GpuMat>.

Rewrite linearPolar & logPolar so that they do not depend on the
deprecated API CvMat. Issue 6377 is resolved in this way because the two
routines do not convert UMat to CvMat anymore.