WebAssembly: (screens) Is the web document a virtual desktop?

Or: does QScreen geometry start at (0,0) or the canvas (x,y) position?

[On Qt for WebAssembly, “screen” does not map to a physical screen, but rather to an
html element on the page - typically a canvas. The application controls the layout of the screens, and places window(s) on screen,]

Using the virtual desktop concept, this results in two modes:

1. Each screen is an independent virtual desktop, where each screen QScreen is positioned (0,0). A window positioned at (0,0) is on its screen. The canvas's page position is not reflected by QScreen.
2. All screens is a part of the same virtual desktop, where the position of each QScreen corresponds to the position of the canvas in the document. Windows should be positioned relative to the target screen's position. (0,0) may be on a different screen than you expect, or outside of any screen.

We can support both modes in the platform plugin, and then select a default, and/or add API for setting it.

There are two cases where its does not matter which mode is in use:

• There is one canvas at (0,0) on the page.
• Each screen has a fullscreen window

These two arguably cover the majority of the use cases, so which mode we select is maybe less important, as long as we are consistent.