Use the `expand` stretch aspect to support arbitrary aspect ratios

The menu and game no longer have black bars when using an aspect ratio
wider than 16:9. This works both with native and sub-native resolutions.

level/level.gd

@@ -49,13 +49,13 @@ func _ready():
 		pass
 	elif Settings.resolution == Settings.Resolution.RES_1080:
 		var minsize = Vector2(window_size.x * 1080 / window_size.y, 1080.0)
-		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_KEEP_HEIGHT, minsize)
+		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_EXPAND, minsize)
 	elif Settings.resolution == Settings.Resolution.RES_720:
 		var minsize = Vector2(window_size.x * 720 / window_size.y, 720.0)
-		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_KEEP_HEIGHT, minsize)
+		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_EXPAND, minsize)
 	elif Settings.resolution == Settings.Resolution.RES_540:
 		var minsize = Vector2(window_size.x * 540 / window_size.y, 540.0)
-		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_KEEP_HEIGHT, minsize)
+		get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_VIEWPORT, SceneTree.STRETCH_ASPECT_EXPAND, minsize)
 
 
 func _input(event):

menu/menu.gd

@@ -54,7 +54,7 @@ onready var loading_progress = loading.get_node(@"Progress")
 onready var loading_done_timer = loading.get_node(@"DoneTimer")
 
 func _ready():
-	get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_2D, SceneTree.STRETCH_ASPECT_KEEP, Vector2(1920, 1080))
+	get_tree().set_screen_stretch(SceneTree.STRETCH_MODE_2D, SceneTree.STRETCH_ASPECT_EXPAND, Vector2(1920, 1080))
 	play_button.grab_focus()
 	var sound_effects = $BackgroundCache/RedRobot/SoundEffects
 	for child in sound_effects.get_children():

project.godot

@@ -35,7 +35,7 @@ window/size/width=1920
 window/size/height=1080
 window/size/fullscreen=true
 window/stretch/mode="2d"
-window/stretch/aspect="keep_width"
+window/stretch/aspect="expand"
 
 [input]