CI: Automate testing via VNC backend

[jserv]

With the implementation of the VNC-compatible backend (#63) for Mado, it serves as a minimalist backend for purposes such as conducting memory analysis with Valgrind on Linux, without the need to start in Linux framebuffer console. VNCSnapshot is a command-line utility for VNC, available from RealVNC and others, that allows taking a snapshot from a VNC server and saving it as a JPEG file.

We can automate testing by first launching the VNC backend and then using VNCSnapshot to take screenshots after connecting to the VNC backend of the running Mado instance. This process will enable us to check the screenshots for consistent functionality and pixel quality on GitHub Actions every time a new pull request is submitted.