Add wxGraphicsContext::GetWindow()

This method allows to retrieve the window this context is associated
with, if any.

Add "wxWindow*" argument to wxGraphicsContext ctor and provide the
window pointer to it when available, i.e. when creating the context from
a wxWindow directly or from wxWindowDC, which is also associated with a
window, in platform-specific code.

No real changes yet.

include/wx/graphics.h

@@ -452,7 +452,7 @@ private:
 class WXDLLIMPEXP_CORE wxGraphicsContext : public wxGraphicsObject
 {
 public:
-    wxGraphicsContext(wxGraphicsRenderer* renderer);
+    wxGraphicsContext(wxGraphicsRenderer* renderer, wxWindow* window = NULL);
 
     virtual ~wxGraphicsContext();
 
@@ -490,6 +490,9 @@ public:
     // create a context that can be used for measuring texts only, no drawing allowed
     static wxGraphicsContext * Create();
 
+    // Return the window this context is associated with, if any.
+    wxWindow* GetWindow() const { return m_window; }
+
     // begin a new document (relevant only for printing / pdf etc) if there is a progress dialog, message will be shown
     virtual bool StartDoc( const wxString& message );
 
@@ -788,6 +791,12 @@ protected:
                                          wxDouble angle,
                                          const wxGraphicsBrush& backgroundBrush);
 
+private:
+    // The associated window, if any, i.e. if one was passed directly to
+    // Create() or the associated window of the wxDC this context was created
+    // from.
+    wxWindow* const m_window;
+
     wxDECLARE_NO_COPY_CLASS(wxGraphicsContext);
     wxDECLARE_ABSTRACT_CLASS(wxGraphicsContext);
 };