Add support for background-color style to span element in wxHTML.

Add code for setting/restoring background mode and use it to implement support
for changing the text background colour.

Closes #14443.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@72589 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/html/htmlcell.h

@@ -91,10 +91,13 @@ public:
     const wxColour& GetFgColour() const { return m_fgColour; }
     void SetBgColour(const wxColour& c) { m_bgColour = c; }
     const wxColour& GetBgColour() const { return m_bgColour; }
+    void SetBgMode(int m) { m_bgMode = m; }
+    int GetBgMode() const { return m_bgMode; }
 
 private:
     wxHtmlSelectionState  m_selState;
     wxColour              m_fgColour, m_bgColour;
+    int                   m_bgMode;
 };