Investigate whole-desktop zooming

Right now the zoom_desktop feature zooms a single output. On xfwm4, it zooms the entire desktop, across all outputs.

Zooming a single output is fairly simple with smithay, as it has first class support for zooming and relocating render elements on screen.

Zooming the entire desktop is hard, because what needs to be done is render every output into a giant offscreen buffer that is the size of all outputs together, and then we have to blit a region (around the zoom point) to the output with the pointer, and all other outputs.

This is especially difficult because Smithay's architecture assumes that you're drawing per-output, which makes sense from a rendering & scanout point of view.

It's probably doable, but is a bit more involved. I actually like the idea of per-output zooming, so if I do decide to implement the full-desktop zoom, I might keep both around as options the user can select.