Allow passing already parsed HTML to wxHtmlDCRenderer

This is more efficient when the same HTML is reused multiple times, e.g.
for measuring it first and then rendering it.

The new function also makes it simpler to parse HTML and manipulate it
in some way before measuring and rendering it.

include/wx/html/htmprint.h

@@ -54,6 +54,11 @@ public:
     // (see wxFileSystem for detailed explanation)
     void SetHtmlText(const wxString& html, const wxString& basepath = wxEmptyString, bool isdir = true);
 
+    // Sets the HTML cell that will be rendered: this is more efficient than
+    // using text as it allows to parse it only once. Note that the cell will
+    // be modified by this call.
+    void SetHtmlCell(wxHtmlContainerCell& cell);
+
     // Sets fonts to be used when displaying HTML page. (if size null then default sizes used).
     void SetFonts(const wxString& normal_face, const wxString& fixed_face, const int *sizes = NULL);
 
@@ -79,11 +84,14 @@ public:
     int GetTotalHeight() const;
 
 private:
+    void DoSetHtmlCell(wxHtmlContainerCell* cell);
+
     wxDC *m_DC;
     wxHtmlWinParser *m_Parser;
     wxFileSystem *m_FS;
     wxHtmlContainerCell *m_Cells;
     int m_Width, m_Height;
+    bool m_ownsCells;
 
     wxDECLARE_NO_COPY_CLASS(wxHtmlDCRenderer);
 };