Implement a new layout mode where we use both the global UI scaling and logical
sized monitors with inverse randr scaling.

It allows to support fractional scaling in the X11 backend with some good
quality image results also in multi-monitor with multi-DPI.

It works by scaling up the UI to the maximum ceiled scaling value across the
monitors, and then scaling down (via randr transformations) the monitors so
that:
  monitor_scaling = ui_scale * monitor_randr_scaling

In order to do this the layout GLOBAL_UI_LOGICAL is added which is logical mode
where monitor positioning is computed dividing the monitor scale by the maximum
scaling across the displays.

This is known to work at best with modesetting drivers when glamor is used.