Simply produce the result into the output image provided by the base
class. This avoid useless copy. This also removes unchecked buffer map
and ensure that GstVideoMeta is respected (for stride mostly).
https://bugzilla.gnome.org/show_bug.cgi?id=775377
Update edgedetect to inherit from GstOpencvVideoFilter instead of from
GstElement. This means less code and more uniformity with other OpenCV
elements.
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.
