Update opencvtextoverlay to inherit from GstOpencvVideoFilter instead of
from GstElement. This means less code and more uniformity with other OpenCV
elements. The chain/transform function is now a third of the size than
before.
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
