Add missing GetVirtualScreenSize stub to HeadlessWrapper (#1789)

Launch.lua calls GetVirtualScreenSize() in DrawPopup() and during
restart, but HeadlessWrapper.lua only stubs GetScreenSize() and
GetScreenScale(). When running headless and an error triggers
DrawPopup on the first OnFrame, this causes a crash because
GetVirtualScreenSize is not yet defined (Modules/Common.lua where
it is normally defined hasn't loaded yet).

The stub delegates to GetScreenSize() since GetScreenScale() returns
1 (no scaling) in headless mode, matching what the real implementation
in Common.lua would produce.

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: YangLeoZhao

src/HeadlessWrapper.lua

@@ -51,6 +51,9 @@ end
 function GetScreenScale()
 	return 1
 end
+function GetVirtualScreenSize()
+	return GetScreenSize()
+end
 function GetDPIScaleOverridePercent()
 	return 1
 end