Matx types do not work inside Mat_ template matrices (Bug #1705)

Types like Matx31f do not work correctly as Mat_<Matx31f> matrices. Not enough memory is allocated for the elements and step from one row to another is miscalculated. Problem exists at least in version 2.3.1a.

Attached test program prints out basic information and element positions in memory for couple of small test matrices. (Warning: Because of this OpenCV bug, it accesses memory outside matrix bounds) All matrices have the same size dimensions (width = 2, height = 5). Attached program gives me the following results:

For cv::Mat_<float>: (correct)
Size: 2 x 5 channels 1 element size: 4 step1: 2 total: 10
Allocated memory: 40 bytes
Pointer to element 0, 0: 20813584
Pointer to element 1, 0: 20813588
Pointer to element 0, 1: 20813592
Pointer to element 1, 1: 20813596
...

For cv::Mat_<cv::Vec3f>: (correct)
Size: 2 x 5 channels 3 element size: 12 step1: 6 total: 10
Allocated memory: 120 bytes
Pointer to element 0, 0: 20818592
Pointer to element 1, 0: 20818604
Pointer to element 0, 1: 20818616
Pointer to element 1, 1: 20818628
...

For cv::Mat_<cv::Matx31f>: (wrong)
Size: 2 x 5 channels 1 element size: 12 step1: 1 total: 10
Allocated memory: 80 bytes
Pointer to element 0, 0: 20818592
Pointer to element 1, 0: 20818604
Pointer to element 0, 1: 20818608
Pointer to element 1, 1: 20818620
...

For cv::Mat_<cv::Matx41f>: (wrong)
Size: 2 x 5 channels 1 element size: 16 step1: 1 total: 10
Allocated memory: 80 bytes
Pointer to element 0, 0: 20818720
Pointer to element 1, 0: 20818736
Pointer to element 0, 1: 20818736
Pointer to element 1, 1: 20818752
...

etc.