backend-vnc: add VNC support using Neat VNC library

This adds basic VNC protocol support using the Neat VNC library (https://github.com/any1/neatvnc). This implementation does not support authentication and hardcodes the pixel format currently. It also does more copying then necessary at this point.

Signed-off-by: Stefan Agner stefan@agner.ch

This is a new attempt in trying to implement a (upstreamable) version of the VNC backend (see my previous attempt !279 (closed), I decided to create a new merge request as this is a different approach). This time I make use of the liberally license library Neat VNC which I only found recently.

There is still some stuff to iron out, most notably I have issues using libuv together with wl_event_loop. My current approach tries to embed the libuv event loop into the wl_event_loop. However, this seems not to work properly somehow, i have to do two calls and the return value of uv_run always claims that there is more work to do... Any hints/ideas welcome!

Other problems are still the same as in !279 (closed), e.g. the keysym to keycode table is still necessary.