Replaced GraphicsHDC from src/msw/renderer.cpp with wxDC::GetTempHDC().

wxDC::GetTempHDC() method provides a convenient and safe way to retrieve HDC
from a wxDC object, whether it is using GDI or GDI+. It is implemented using
(MSW-specific) virtual functions in wxDC and so doesn't need ugly hacks like
wxDynamicCast which were used in src/msw/renderer.cpp to achieve the same
effect.

Also, we now use GetTempHDC() consistently in all wxMSW rendering methods as
the old GraphicsHDC was only used in some of them meaning that many methods
didn't work at all with wxGCDC.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@62294 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775

include/wx/dcgraph.h

@@ -35,6 +35,14 @@ public:
     wxGraphicsContext* GetGraphicsContext();
     void SetGraphicsContext( wxGraphicsContext* ctx );
 
+#ifdef __WXMSW__
+    // override wxDC virtual functions to provide access to HDC associated with
+    // this Graphics object (implemented in src/msw/graphics.cpp)
+    virtual WXHDC AcquireHDC();
+    virtual void ReleaseHDC(WXHDC hdc);
+#endif // __WXMSW__
+
+private:
     DECLARE_DYNAMIC_CLASS(wxGCDC)
     wxDECLARE_NO_COPY_CLASS(wxGCDC);
 };