opencv: fix caps issues and extend supported caps for some elements

opencv: fix caps issues and extend supported caps for some elements

Some elements had vague caps, such as "video/x-raw-rgb", which caused problems
at least with textwrite. For other elements, the underlying OpenCV functions
support more than just one image type, so I increased the number of supported
caps.

I created a utility function "gst_opencv_caps_from_cv_image_type", so each
element creates caps directly from OpenCV image types, such as CV_8UC1 for
8-bit grayscale. This function uses gstvideo to create uniform caps.

https://bugzilla.gnome.org/show_bug.cgi?id=635304