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
Some platforms such as Wayland need to set environment vairables before
SDL is initialized, as such this change detects the display server
before SDL has started and calls the new `earlyInit` method providing
the implementation an opportunity to set things up.
