decoration: add NON_SOLID flag for shadow

fixes #3841

src/render/decorations/IHyprWindowDecoration.hpp

@@ -24,6 +24,7 @@ enum eDecorationFlags
 {
     DECORATION_ALLOWS_MOUSE_INPUT  = 1 << 0, /* this decoration accepts mouse input */
     DECORATION_PART_OF_MAIN_WINDOW = 1 << 1, /* this decoration is a *seamless* part of the main window, so stuff like shadows will include it */
+    DECORATION_NON_SOLID           = 1 << 2, /* this decoration is not solid. Other decorations should draw on top of it. Example: shadow */
 };
 
 class CWindow;