Viewports: fix missing default per-window value for ParentViewportId due to zero-cleared in-window instance (#4756)

Broken by 2080d12b

imgui.cpp

@@ -3092,6 +3092,7 @@ ImGuiWindow::ImGuiWindow(ImGuiContext* context, const char* name) : DrawListInst
     DrawList = &DrawListInst;
     DrawList->_Data = &context->DrawListSharedData;
     DrawList->_OwnerName = Name;
+    IM_PLACEMENT_NEW(&WindowClass) ImGuiWindowClass();
 }
 
 ImGuiWindow::~ImGuiWindow()