AC_CHECK_HEADERS() calls action-if-not-found also if just one of
the headers checked for is missing, which is not what we wanted.
Also, check for highgui_c.h instead of highgui.hpp.
https://bugzilla.gnome.org/show_bug.cgi?id=672226
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
