This commit adds support for rendering onto enlarged per logical
monitor framebuffers, using the scaled clutter stage views, for HiDPI
enabled logical monitors.

This works by scaling the mode of the monitors in a logical monitors by
the scale, no longer relying on scaling the window actors and window
geometry for making windows have the correct size on HiDPI monitors.

It is disabled by default, as in automatically created configurations
will still use the old mode. This is partly because Xwayland clients
will not yet work good enough to make it feasible.

To enable, add the 'scale-monitor-framebuffer' keyword to the
org.gnome.mutter.experimental-features gsettings array.

It is still possible to specify the mode via the new D-Bus API, which
has been adapted.

The adaptations to the D-Bus API means the caller need to be aware of
how to position logical monitors on the stage grid. This depends on the
'layout-mode' property that is used (see the DisplayConfig D-Bus
documentation).

https://bugzilla.gnome.org/show_bug.cgi?id=777732