yarp::sig::file::write() doesn't handle properly images retrieved with yarp::cv::fromCvMat()

[pattacini]

Describe the bug
Following up #1932, I've updated the test yarp/example/opencv that now uses the latest yarp::cv::[toCvMat|fromCvMat<>] routines handling the conversion [to|from] OpenCV.

Two test images are saved by means of yarp::sig::file::write() function, which should be the same but they're not.

To Reproduce

• cmake
• make
• launch the binary
• compare test-1.ppm vs. test-2.ppm

Expected behavior
The two images should look the same.

Screenshots

test-1.ppm	test-2.ppm

Note that the two images have been converted into png to be dropped herein.

Configuration:

• OS: Windows
• yarp version: current devel
• compiler: MSVC 2017

cc @Nicogene @drdanz

[pattacini]

Actually, df586cc solves the problem.

There was only one memory area (allocated by yarpImage) that was wrapped twice causing the bug.