neatvnc: Disable the CPU acceleration by default

For Nixpkgs it seems best to disable the usage of CPU extensions by
default as older CPUs don't support them which will result in a crash at
runtime (SIGILL: Illegal instruction) [0].
The performance on my old test system was more than enough anyway
(usually ~10% CPU usage due to wayvnc).

[0]: https://github.com/any1/neatvnc/issues/21