[meta] Improve Servo initialisation

[paulrouget]

This issue is part of an effort to improve Servo's embedding story. See https://github.com/paulrouget/servoshell/projects/2

At the moment, starting multiple instances of Servo will have 3 major issues:

• they all have to share the same initial URL because the URL is set as an option which is a global object
• beside the latest instance, pages are not rendered (black screen)
• there are as constellations as there are browsers

At least we need:

• Make opts not global, and/or:
• Initial URL should be decoupled from the options
• We should first start a constellation, and then attach browsers to it (#13994)
• be able to switch GL contexts
• Constellation should not assume one compositor (#13993)

[glennw]

These are some relevant WR issues:

• servo/webrender#771 (this is being worked on).
• servo/webrender#933

[paulrouget]

I'm going to close this as most of it is done now. Only multi-window support requires initialization related work. See #13993.