OpenCV mask operator in FittingFilters::fillInvalidAreas is not properly implemented for newer OpenCV versions

[M. Gronle]

The current implementation to get a mask of all valid values in fillInvalidAreas is not properly working for newer OpenCV versions (e.g. 4.5.0):

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The equal operator to get a mask of NaN values was working in earlier versions of OpenCV, but has to be replaced by a robust version.

[M. Gronle]

A unittest has been added to the unittest_dataobject (miscellaneous.cpp) to verify, if cv::Mat == cv::Mat will return an uint8 mask, where all values are set to 255 besides NaN (set to 0). This unittest returned true, such that this issue is no problem.

The bug report https://github.com/opencv/opencv/issues/16465 also states, that the == operator returns the expected behaviour, but the != operator does not. This could be verified in the new unittest.