Fix a memory leak of ImGuiColumnsSet's Columns vector.  ImVector doesn't call destructors.
diff --git a/imgui.cpp b/imgui.cpp
index b9ceb11..2b6b699 100644
--- a/imgui.cpp
+++ b/imgui.cpp
@@ -1876,6 +1876,8 @@
 {
     IM_DELETE(DrawList);
     IM_DELETE(Name);
+    for (int i = 0; i != ColumnsStorage.Size; i++)
+        ColumnsStorage[i].~ImGuiColumnsSet();
 }
 
 ImGuiID ImGuiWindow::GetID(const char* str, const char* str_end)