video: Rename gst_video_color_transfer_*() to gst_video_transfer_function_*() in...

video: Rename gst_video_color_transfer_*() to gst_video_transfer_function_*() in new API

The type is called GstVideoTransferFunction so the function names should
match, otherwise gobject-introspection is keeping the functions as
global functions instead of methods on the type.

The same mistake was also made in lots of other APIs over the years, but
here we can at least fix it for 1.18 still.

Thanks to Marijn Suijten for noticing.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/807>