Add GetVirtualScreenSize for headless mode

Author: vaisest

src/HeadlessWrapper.lua

@@ -7,6 +7,9 @@
 -- bodies intended for headless use.
 dofile("_SimpleGraphic.def.lua")
 
+function GetVirtualScreenSize()
+	return 1920, 1080
+end
 
 -- Callbacks
 __callbackTable__ = { }