MP Create Game: keep background image width proportional to its height

[ci skip]

(cherry-picked from commit 604dcd8008)

data/gui/window/mp_create_game.cfg

@@ -1016,7 +1016,7 @@
 		[image]
 			x = "(width - image_width)"
 			y = 0
-			w = "(image_width)"
+			w = "(image_original_width * (height / image_original_height))"
 			h = "(height)"
 
 			# TODO: this should be generated with IPFs once we get a gradient function