We can only have 1 single GstTranscoderSignalAdapter object for a
given GstTranscoder object, this enforces that by avoiding to expose
a constructor and instead add a method to GstTranscoder to get the
signal adapter (internally creating it when needed). We can still
cleanly ensure that the signal adapter is running for the requested
GMainContext and return NULL if it is not the case.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/2044>
Instead of implementing exactly the same thing ourself but making
`GstBus` not know that it is the case.
Since we are *sure* that the bus can't have been access at the point
where we add the watch we are guaranteed that the current thread
maincontext is going to be used.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-bad/-/merge_requests/1870>
