Initialize wxHtmlSelectionState::m_bgMode in ctor

Don't leave the variable uninitialized to avoid warnings from static code
analyzer and to ensure that it is never used before being initialized (while
this does seem to be the case already, it's not exactly obvious).

include/wx/html/htmlcell.h

@@ -81,7 +81,7 @@ enum wxHtmlSelectionState
 class WXDLLIMPEXP_HTML wxHtmlRenderingState
 {
 public:
-    wxHtmlRenderingState() : m_selState(wxHTML_SEL_OUT) {}
+    wxHtmlRenderingState() : m_selState(wxHTML_SEL_OUT) { m_bgMode = wxSOLID; }
 
     void SetSelectionState(wxHtmlSelectionState s) { m_selState = s; }
     wxHtmlSelectionState GetSelectionState() const { return m_selState; }