Improve creating wxGCDC from an existing wxGraphicsContext

Avoid creating a default wxGraphicsContext unnecessarily, only to
immediately delete it and replace it with the provided one.

This was at best unnecessary and at worst resulted in a crash if the
default context couldn't be created, as happened on a headless system
(where wxImage-based wxGraphicsContext can still be created
successfully).

include/wx/dcgraph.h

@@ -60,6 +60,10 @@ public:
 #if defined(__WXMSW__) && wxUSE_ENH_METAFILE
     wxGCDCImpl( wxDC *owner, const wxEnhMetaFileDC& dc );
 #endif
+
+    // Ctor using an existing graphics context given to wxGCDC ctor.
+    wxGCDCImpl(wxDC *owner, wxGraphicsContext* context);
+
     wxGCDCImpl( wxDC *owner );
 
     virtual ~wxGCDCImpl();

src/common/dcgraph.cpp

@@ -106,9 +106,8 @@ wxGCDC::wxGCDC(const wxEnhMetaFileDC& dc)
 #endif
 
 wxGCDC::wxGCDC(wxGraphicsContext* context) :
-    wxDC( new wxGCDCImpl( this ) )
+    wxDC(new wxGCDCImpl(this, context))
 {
-    SetGraphicsContext(context);
 }
 
 wxGCDC::wxGCDC() :
@@ -122,6 +121,12 @@ wxGCDC::~wxGCDC()
 
 wxIMPLEMENT_ABSTRACT_CLASS(wxGCDCImpl, wxDCImpl);
 
+wxGCDCImpl::wxGCDCImpl(wxDC *owner, wxGraphicsContext* context) :
+    wxDCImpl(owner)
+{
+    Init(context);
+}
+
 wxGCDCImpl::wxGCDCImpl( wxDC *owner ) :
    wxDCImpl( owner )
 {