Make wxWindow::GetContentScaleFactor() useful for non-OSX platforms.

Return the ration of the current DPI to the standard one to allow using this
function to scale pixel values for the current screen resolution.

src/common/wincmn.cpp

@@ -791,6 +791,21 @@ wxSize wxWindowBase::DoGetBestSize() const
     return best;
 }
 
+double wxWindowBase::GetContentScaleFactor() const
+{
+    // Currently we don't support per-monitor DPI, so it's useless to construct
+    // a DC associated with this window, just use the global value.
+    //
+    // We also use just the vertical component of the DPI because it's the one
+    // that counts most and, in practice, it's equal to the horizontal one
+    // anyhow.
+    //
+    // Finally, we consider 96 DPI to be the standard value, this is correct
+    // at least for MSW, but could conceivably need adjustment for the other
+    // platforms.
+    return wxScreenDC().GetPPI().y / 96.;
+}
+
 // helper of GetWindowBorderSize(): as many ports don't implement support for
 // wxSYS_BORDER/EDGE_X/Y metrics in their wxSystemSettings, use hard coded
 // fallbacks in this case