re-slice linux/win32 backends, top-level folder is backend (demon/win32 -> win32/demon, etc.); os_gfx -> window_manager; fix linux / opengl builds for new apis/types

src/window_manager/window_manager_stub.h

@@ -0,0 +1,7 @@
+// Copyright (c) Epic Games Tools
+// Licensed under the MIT license (https://opensource.org/license/mit/)
+
+#ifndef OS_GFX_STUB_H
+#define OS_GFX_STUB_H
+
+#endif // OS_GFX_STUB_H