Override HasTransparentBackground() in wxHyperlinkCtrl to return true.

At least in wxMSW the control must override HasTransparentBackground() to
return true if it really wants its background to be transparent, so do it in
wxHyperlinkCtrlBase to fix the background appearance when using the generic
implementation in wxMSW.

See #12271.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@66696 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/hyperlink.h

@@ -68,6 +68,8 @@ public:
     // NOTE: also wxWindow::Set/GetLabel, wxWindow::Set/GetBackgroundColour,
     //       wxWindow::Get/SetFont, wxWindow::Get/SetCursor are important !
 
+    virtual bool HasTransparentBackground() { return true; }
+
 protected:
     virtual wxBorder GetDefaultBorder() const { return wxBORDER_NONE; }