tdf#126788 use singleton to avoid initial allocation

Change-Id: I55011cdd3e6c457c6ab91d90579ace3524a9889d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/142029
Tested-by: Jenkins
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>

basegfx/source/polygon/b2dpolygon.cxx

@@ -1075,7 +1075,10 @@ public:
 
 namespace basegfx
 {
-    B2DPolygon::B2DPolygon() = default;
+    static o3tl::cow_wrapper<ImplB2DPolygon> DEFAULT;
+
+    B2DPolygon::B2DPolygon()
+        : mpPolygon(DEFAULT) {}
 
     B2DPolygon::B2DPolygon(std::initializer_list<basegfx::B2DPoint> aPoints)
     {