backend/wayland: handle hidpi and subpixel

Hi, this is a patch to scale the wayland backend in case it's running on a compositor with screen scaling configured. Should handle multi-output configurations with different scales.

I also threw in a patch for passing through subpixel hints as that became trivial since all the footwork has been done

RFC: We might want to pass the scale further into the system so that nested applications running within know about the actual scale they are running at from the host compositor.

Closes: #854, #1758