PnP solver: fixed element-wise access

Before the big PnP update, the data was stored row-first (n points in one single line). Now, the RansacCallback is used which stores column-first (one single point in n lines). This led to an [debug assertion failing](https://github.com/PhilLab/opencv/blob/pnpFix/modules/core/include/opencv2/core/mat.inl.hpp#L868-L870) when using any other method than SOLVEPNP_ITERATIVE (in release mode, the access did work correctly since the next memory cell contains the next value regardless of column- or row-first). To fix this bug and to be storage-agnostic, we now use the single-paramenter element access (since in all cases we have either one row or one column).