Draft: xwayland: Multi DPI support via global factor rescaling

This is an updated version of MR !111 with some changes. Therefore credit for most of the code goes to the original author @romangg .

The original !111 automatically sets the global scale factor to the maximum scale factor of all outputs. This PR allows the wayland compositor to explicitly set the scale factor via a newly introduced XWAYLAND X protocol instead. This has a some advantages:

• More flexibility for the compositor:
  • If the compositor wants to automatically set the scale to the max of all outputs, it can still do it himself.
  • Alternatively, it can leave the configuration to the user. For example, a user may want to optimize text quality for a lower-DPI monitor.
• The original !111 added a --max-factor-rescale command line flag so that the compositor can advertise support. This flag is no longer needed, because compositors without xwayland scaling support won't set a scale. The default scale is 1, which is equivalent to the old behavior. Therefore this change is backwards compatible without having to add the flag.