Fix Visual Studio compilation with OpenMP

The commit e75a921c8f74e260f4b06b71672507b91f69866c changed the type of some index variables from int to size_t, which triggers a Visual Studio compiler error (error C3016: 'i' : index variable in OpenMP 'for' statement must have signed integral type)

Also changed CV_FM_RANSAC to cv::FM_RANSAC to be independent of OpenCV legacy headers.