Stretch backgrounds based on preference

It turns out we have a preference called StretchBackgrounds and this
seems like the perfect place to use it. Also we may want to set it to
true then by default, I don't know.

(Sourced from shakesoda/stepmania.)

Themes/_fallback/Scripts/02 Actor.lua

@@ -152,7 +152,12 @@ function Actor:FullScreen()
 end
 
 function Actor:scale_or_crop_background()
-	self:cropto(SCREEN_WIDTH, SCREEN_HEIGHT)
+	if PREFSMAN:GetPreference("StretchBackgrounds") then
+		self:cropto(SCREEN_WIDTH, SCREEN_HEIGHT)
+	else
+		local graphicAspect = self:GetWidth()/self:GetHeight()
+		self:zoomto(SCREEN_HEIGHT*graphicAspect,SCREEN_HEIGHT)
+	end
 end
 
 function Actor:CenterX() self:x(SCREEN_CENTER_X) end