scene: move source to subdir

This will allow more scene-graph extensions to be added without
cluttering wlr_scene.c, for instance for sub-surface handling and
wlr_output_layout integration.

types/meson.build

@@ -3,6 +3,7 @@ wlr_files += files(
 	'data_device/wlr_data_offer.c',
 	'data_device/wlr_data_source.c',
 	'data_device/wlr_drag.c',
+	'scene/wlr_scene.c',
 	'seat/wlr_seat_keyboard.c',
 	'seat/wlr_seat_pointer.c',
 	'seat/wlr_seat_touch.c',
@@ -49,7 +50,6 @@ wlr_files += files(
 	'wlr_primary_selection.c',
 	'wlr_region.c',
 	'wlr_relative_pointer_v1.c',
-	'wlr_scene.c',
 	'wlr_screencopy_v1.c',
 	'wlr_server_decoration.c',
 	'wlr_surface.c',