compositor-drm: Allow weston_output_mode_switch_to_native() to work.

Initialize output->native_mode with the initially chosen
mode for an output, so weston_output_mode_switch_to_native()
has something to work with and can switch back from temporary
selected modes to the outputs native mode. Before, this was a
no-op.

This allows an output to switch back to its default mode if
a former toplevel fullscreen shell surface created via method
WL_SHELL_SURFACE_FULLSCREEN_METHOD_DRIVER gets destroyed, or
it gets demoted to non-fullscreen, or if modesetting on the
output failed for some reason.

v2: Modified and split into a separate patch from original
    patch "Allow restore_output_mode() to work properly.",
    as suggested by Derek Foreman.

Signed-off-by: Mario Kleiner <mario.kleiner.de@gmail.com>
Cc: Derek Foreman <derekf@osg.samsung.com>
Reviewed-By: Derek Foreman <derekf@osg.samsung.com>

src/compositor-drm.c

@@ -2202,6 +2202,10 @@ create_output_for_connector(struct drm_compositor *ec,
 				    connector->count_modes == 0 ?
 				    ", built-in" : "");
 
+	/* Set native_ fields, so weston_output_mode_switch_to_native() works */
+	output->base.native_mode = output->base.current_mode;
+	output->base.native_scale = output->base.current_scale;
+
 	return 0;
 
 err_output: