Implement wayland backend cursor

Branched this off the style issue fix branch by accident, can rebase it as soon as that is merged. Tested on weston and sway. Had to add os-compatibility.c for the shm buffer (stripped everything unneeded).

There is one (minor) issue, i had to use a void* instead of wl_buffer* in the wlr_wl_backend_output since otherwise the compiler warns (and therefore errors) about wl_buffer being deprecated (since there are wl_buffer structs in the client and server side, deprecated on server side, we actually refer to the client side definition here...). Not sure if this is a problem or if it can be handled better. Resolves #46 (closed).