In their base init function, they must add pad templates for the sink
pad and the source pad to the element class, describing the media type
they accept and output in the caps of the pad template.
In their class init function, they must override the
GST_TAG_MUX_CLASS(mux_klass)->render_start_tag and/or
GST_TAG_MUX_CLASS(mux_klass)->render_end_tag vfuncs and set up a render
function.
Provides a base class for adding tags at the beginning or end of a stream.
Deriving from GstTagMux
Subclasses have to do the following things: