This work was motivated by libevdev, which too started out with a global log
handler, and that's biting us now. libevdev is lower-level than libinput, so
there's a higher chance of two unrelated bits loading libevdev into the same
process. Nonetheless, having a global log handler in a library makes me a
bit nervous now.
This patchset changes over to a per-context log handler and that breaks the
API in a couple of ways:
* the libinput context is now passed into the log function
* the user_data pointer was dropped, use the context's user data if needed
* the log handler/priority is now part of the libinput_interface and managed
by the caller, not by libinput (hence all the libinput_set/get_log*
functions are gone)
* the udev context creation was split up so we can give the client a
context before calling a log function that uses that context. I've left
the old function there for now for some backwards compatibility but tbh
given that everything else changes I think we should just drop
libinput_udev_create_for_seat().
Cheers,
Peter