cvarrToMat() is added because it is compatible with all versions of Opencv
and using the class Mat constructor is eliminated, because is deprecated
in 3.X versions. The use the using namespace cv because is called some
functions many times. This keeps compatibility with 2.4.
https://bugzilla.gnome.org/show_bug.cgi?id=760473
The opencv element includes were full of duplicates and uneeded headers.
For example a few elements that stopped using gstcvopencvutils still
included that header file.
If old opencv1-style legacy include directory is available,
this change becomes purely cosmetic (maybe will compile a bit faster).
It becomes an FTBFS fix when opencv1-style include directory is missing
(possibly because opencv package maintainer decided not to pack it).
https://bugzilla.gnome.org/show_bug.cgi?id=747705
