Thibault Saunier authored Dec 10, 2020 and GStreamer Marge Bot committed Dec 14, 2020

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: <!1870>