Avoid duplicated surf extraction step when same parameters are used for detection and descriptors calculation

added first version of public stitching API, added simple sample which uses that API, old sample renamed to stitching_detailed

switched to gpu::remap in opencv_stitching

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

fixed bug in opencv_stitching (corrected resize images step), added matches checking (both 1->2 and 2->1 must be presented)

fixed error in gcgraph which led to problems in opencv_stitching GC seam finder, added check for duplication in matches (opencv_stitching)