implement Flush() for wxImage wxGraphicsContext under Cairo and GDI+, see #16694

src/generic/graphicc.cpp

@@ -510,6 +510,11 @@ public:
     }
 
     virtual ~wxCairoImageContext()
+    {
+        Flush();
+    }
+
+    virtual void Flush() wxOVERRIDE
     {
         m_image = m_data.ConvertToImage();
     }

src/msw/graphics.cpp

@@ -451,6 +451,11 @@ public:
     }
 
     virtual ~wxGDIPlusImageContext()
+    {
+        Flush();
+    }
+
+    virtual void Flush() wxOVERRIDE
     {
         m_image = m_bitmap.ConvertToImage();
     }