Servo improvements for Tauri

Verso is a new browser initiative that is based on the Servo browser engine - a cross-platform, open source web engine written in Rust managed by Linux Foundation Europe. The project originates from an earlier effort to integrate Servo in Tauri, a widely used open source system for distributing cross-platform applications capable of running content and applications using web technology outside of the browser. The web ecosystem currently lacks a cross-platform, non-corporate controlled system for doing so, meaning that solutions like Tauri need to rely on the platform engines controlled by Apple, Google, and Microsoft. Obviously, this add complexity, has security and stability implications, lacks consistency, and involves limited levels of user agency. Integrating a portable browser engine would be a major step towards being able to run applications in a consistent, open source web runtime on major desktop and mobile platforms.

As part of that work, it became clear that several improvements to Servo are urgently needed. In order to speed up the development of those improvement, it turned out to be more efficient to transpose these requirements to a new standalone browser: Verso. The key tasks beyond improving developer efficiency and workflow (also for Mozjs and Spidermonkey) tackled in this project are offscreen rendering and multiwebview support.