Merge branch 'gcdc-without-display'

Make it possible to use wxGCDC without a display.

See https://github.com/wxWidgets/wxWidgets/pull/1044

include/wx/dc.h

@@ -686,6 +686,16 @@ protected:
     // for rendering on higher-resolution DCs such as printer ones
     static float GetFontPointSizeAdjustment(float dpi);
 
+    // Return the number of pixels per mm in the horizontal and vertical
+    // directions, respectively.
+    //
+    // If the physical size of the DC is not known, or doesn't make sense, as
+    // for a SVG DC, for example, a fixed value corresponding to the standard
+    // DPI is used.
+    double GetMMToPXx() const;
+    double GetMMToPXy() const;
+
+
     // window on which the DC draws or NULL
     wxWindow   *m_window;
 
@@ -715,9 +725,12 @@ protected:
     
     double m_contentScaleFactor; // used by high resolution displays (retina)
 
-    // what is a mm on a screen you don't know the size of?
-    double       m_mm_to_pix_x,
-                 m_mm_to_pix_y;
+    // Pixel per mm in horizontal and vertical directions.
+    //
+    // These variables are computed on demand by GetMMToPX[xy]() functions,
+    // don't access them directly other than for assigning to them.
+    mutable double m_mm_to_pix_x,
+                   m_mm_to_pix_y;
 
     // bounding and clipping boxes
     wxCoord m_minX, m_minY, m_maxX, m_maxY; // Bounding box is stored in device units.

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();

include/wx/dfb/dc.h

@@ -157,8 +157,6 @@ private:
 protected:
     wxIDirectFBSurfacePtr m_surface;
 
-    double            m_mm_to_pix_x, m_mm_to_pix_y;
-
     friend class WXDLLIMPEXP_FWD_CORE wxOverlayImpl; // for Init
 
     wxDECLARE_ABSTRACT_CLASS(wxDFBDCImpl);