lokdocview: ensure private structure is allocated with operator new

It's undesirable to malloc a struct that has a TileBuffer member, while
TileBuffer doesn't have a default ctor.

Change-Id: I72dfacc0088f238ee101d84838bd7eea51ced82a

libreofficekit/source/gtk/tilebuffer.hxx

@@ -80,8 +80,8 @@ private:
 class TileBuffer
 {
  public:
- TileBuffer(LibreOfficeKitDocument *document,
-            int columns)
+ TileBuffer(LibreOfficeKitDocument *document = 0,
+            int columns = 0)
      : m_pLOKDocument(document)
         , m_nWidth(columns)
     {