Fix regression in wxGCDC initialization from wxGraphicsContext

The changes of ae2cb7d347 resulted in
transformation matrix not being initialized correctly any more.

Fix this by adding yet another helper, DoInitContext(), called both when
initializing wxGCDC using an existing wxGraphicsContext in the
corresponding ctor and the just created one in the other ones.

Closes #18429.

include/wx/dcgraph.h

@@ -242,6 +242,10 @@ private:
     // is assumed to be newly created.
     void Init(wxGraphicsContext*);
 
+    // This method initializes m_graphicContext, m_ok and m_matrixOriginal
+    // fields, returns true if the context was valid.
+    bool DoInitContext(wxGraphicsContext* ctx);
+
     wxDECLARE_CLASS(wxGCDCImpl);
     wxDECLARE_NO_COPY_CLASS(wxGCDCImpl);
 };