Add wxDisplay::GetPPI() to use instead of wxGetDisplayPPI()

While this is not done for all the ports yet, the new API allows
returning different PPI values for different monitors, unlike the old
(and still existing, but implemented in terms of the new one) global
function.

include/wx/private/display.h

@@ -81,6 +81,15 @@ public:
     // return the depth or 0 if unknown
     virtual int GetDepth() const = 0;
 
+    // return the resolution of the display, uses GetSizeMM() by default but
+    // can be also overridden directly
+    virtual wxSize GetPPI() const;
+
+    // return the physical size of the display or (0, 0) if unknown: this is
+    // only used by GetPPI() implementation in the base class, so if GetPPI()
+    // is overridden, this one doesn't have to be implemented
+    virtual wxSize GetSizeMM() const { return wxSize(0, 0); }
+
     // return the name (may be empty)
     virtual wxString GetName() const { return wxString(); }