subsurfaceTree: assign surfaces to a CWLSurface

fixes #4872

src/helpers/SubsurfaceTree.cpp

@@ -145,6 +145,7 @@ void Events::listener_newSubsurfaceNode(void* owner, void* data) {
     const auto        PSUBSURFACE = (wlr_subsurface*)data;
 
     const auto        PNEWSUBSURFACE = &pNode->childSubsurfaces.emplace_back();
+    PNEWSUBSURFACE->surface.assign(PSUBSURFACE->surface);
 
     Debug::log(LOG, "Added a new subsurface {:x}", (uintptr_t)PSUBSURFACE);
 

src/helpers/SubsurfaceTree.hpp

@@ -36,6 +36,7 @@ struct SSurfaceTreeNode {
 
 struct SSubsurface {
     wlr_subsurface*   pSubsurface = nullptr;
+    CWLSurface        surface;
 
     SSurfaceTreeNode* pParent = nullptr;
     SSurfaceTreeNode* pChild  = nullptr;