Move SCO polling to a single place, and abstract mtu handling.
Autodetect suitable tx packet size based on rx, instead of relying on
the kernel providing correct values.
Fixespipewire/pipewire#502.
Also move media application creating from filter callback to initialization.
Application object path and endpoint paths only need to be registered once.
Signed-off-by: Huang-Huang Bao <eh5@sokka.cn>
Backends don't necessarily allow for opening the same device multiple
times, and it shouldn't be necessary.
Since source and sink are not necessarily both running at the same time,
refcount the transport acquire/release so that it knows to close the fd
only when no source/sink is running.
Let the transport manage the fd lifecycle, also closing it once it is
not needed.
Don't return the fd from acquire(), since each transport is associated
with a single socket fd.
It seems that some BT adapters don't return correct SCO MTU, see
https://marc.info/?t=148586768600002&r=1&w=2
Do not try to autodetect the MTU and use a fix MTU size of 48 bytes.
