Add wxDisplayImpl::GetScaleFactor() and implement it for wxGTK

We need to account for the scale factor under GTK+ (and, presumably,
under macOS) to compute the correct PPI value as it must use the number
of physical and not logical pixels.

src/common/dpycmn.cpp

@@ -195,9 +195,7 @@ bool wxDisplay::ChangeMode(const wxVideoMode& mode)
 
 wxSize wxDisplayImpl::GetPPI() const
 {
-    const wxSize pixels = GetGeometry().GetSize();
     const wxSize mm = GetSizeMM();
-
     if ( !mm.x || !mm.y )
     {
         // Physical size is unknown, return a special value indicating that we
@@ -205,6 +203,10 @@ wxSize wxDisplayImpl::GetPPI() const
         return wxSize(0, 0);
     }
 
+    // We need physical pixels here, not logical ones returned by
+    // GetGeometry(), to compute the real DPI.
+    const wxSize pixels = GetGeometry().GetSize()*GetScaleFactor();
+
     return wxSize(wxRound((pixels.x * inches2mm) / mm.x),
                   wxRound((pixels.y * inches2mm) / mm.y));
 }