added gpu::buildWarpPlaneMaps and gpu::buildWarpCylindricalMaps functions, integrated into stitching

GPU: updated upsample, downsample functions, added pyrDown, pyrUp, added support of 16S filtering; put spherical warper on GPU (from opencv_stitching)

fixed BFM warning ("cannot tell what pointer points to")

minor optimization of SURF_GPU (reduce memory transfers, use structure of arrays instead of array of structures)

added downsample function into gpu module, refactored it a little bit, added guard for CUDA related include in cascadeclassifier_nvidia_api.cpp

removed test whether z is negative in gpu::solvePnpRansac (there is no need in this for tests passing)

temporarily disabled compute descriptor kernel for new cards (some problems with threads synchronization), old version of kernels is used.

minor fixes and refactoring of GPU module

added assertion after all kernels calls

minor code refactoring