Move rendering mode into GdkDisplay

This is in preparation for making it runtime-settable in the
inspector.
gdk: Add private API for rendering mode

This will be used to expose rendering mode in the inspector.
inspector: Add a rendering mode control
gdk: Make debug-updates a per-display setting

Add private API to set this per-display, and make the existing
gdk_window_set_debug_update function set a global default.
inspector: Only set debug-updates for the default display

Use the new private API to isolate the inspector itself from
this setting.