[Meta] multi windows support

[paulrouget]

Meta bug to track multi window support:

• Support multiple windows #13338
• [multi window] Constellation should not assume there's one and only one compositor #13993
• [multi window] Constellation should be able to handle multiple root frame #13994
• make it possible to configure where to paint a top level pipeline #13995
• Multiwindow example is broken on OS X rust-windowing/winit#56

[paulrouget]

There's most likely a lot of work to be done. But I think we should start by suppressing the assumption that there is only one root frame, and only one compositor, and that a top level pipeline is drawn full window.

[paulrouget]

Here is how I imagine this would work (as a first step):

• supports command line: servo http://site1.com http://site2.com (opening new windows from window.open will come later)
• in servo/main:
  • create 2 glutin windows, 2 compositors, 1 constellation with 2 browsers
  • spawn a thread for each window event handler
  • wait until both windows close to exit
• in constellation,
  • remove root_frame_id
  • add a map root_frame <-> compositor
  • sort out compositor messages (find right frame for compositor, for example, key events)

I'm sure there are plenty of other things to take into consideration.

I have no idea how this will impact the compositor code.

[paulrouget]

More about compositor / webrender implications here: #13338

[paulrouget]

We will also need to fix glutin/winit to support multiwindow. See rust-windowing/winit#56

[pshaughn]

WPT xhr/open-url-multi-window-6.htm cares (probably among many other WPT tests I haven't dug into yet)