Unsupported backends were rejected when we try to initialize them, now
we reject them in the option parsing phase.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
leftovers:
1) config file path. Has to implement compatibility functionalities before
we can change it.
2) links in man pages. Has to migrate the repo first.
3) _COMPTON_SHADOW, it has become a defacto standard, so we have to keep
supporting it.
4) dbus names, undecided whether we should/could change it.
Signed-off-by: Yuxuan Shui <yshuiv7@gmail.com>
