add multi-window support

[igagis]

Multiwindow support added for:

• Windows
• Linux (Wayland/xorg)
• Macos

Still need to do:

• divide main loop cycle time between windows? Call updater.update() after rendering each window? Currently, e.g. in Linux xorg, when two or more windows open, it synchronizes to VSYNC for rendering each window, so the FPS effectively divided by number of windows.
• In Linux Wayland, the rendering happens with no sync to the main loop iterations, so mainloop can run as busy loop if there is an active updateable while rendering is done when Wayland sends the render event. Need to sync the rendering with the main loop cycle somehow?