getcaps is called frequently during stream setup, and creating a new
stream each time is very inefficient. There's some more room for
optimisation by caching the queried sink formats as well, but this needs
some more changes to listen for format changes on the sink (for when
supported formats change between probe stream creation and sink
querying).
https://bugzilla.gnome.org/show_bug.cgi?id=686459
c7e65777e1