zwp_relative_pointer_manager_v1 and zwp_pointer_constraints_v1 are
supported by GNOME/KDE/sway (and most other compositors), but they are
not a required part of the protocol.
Some users also run software in one-off nested compositors like cage[0]
for an extra layer of isolation; cage, at least, does not support
pointer captures.
This commit makes Looking Glass warn when an optional protocol is
unsupported, and fail if a required one is missing. Pointer grab paths
have a new guard against the aforementioned protocols being missing.
[0]: https://github.com/Hjdskes/cage
