Compute wxDCImpl::m_mm_to_pix_[xy] on demand

If nothing else, this avoids 2 calls to each of wxGetDisplaySize() and
wxGetDisplaySizeMM() on each and every wxGCDC construction which are
completely unnecessary as wxGCDCImpl uses its own hardcoded resolution
of 72 DPI, as do some other classes deriving from wxDCImpl.

And even for the classes which do compute these fields using the display
resolution, it may still be unnecessary to do it as they can be never
used if neither GetSizeMM() nor SetLogicalScale() are called on the
corresponding wxDC object.

Finally, this is more than an optimization as when using Cairo-based
wxGCDC without display (which is explicitly supported), calling
wxGetDisplaySize() simply doesn't work at all.

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.