Add wxDisplay::GetScaleFactor()

This is conceptually the same as the ratio of the current DPI to the
standard one, but can be implemented more directly for wxGTK3 and wxOSX
(although the latter doesn't implement it yet).

src/common/dpycmn.cpp

@@ -149,6 +149,25 @@ wxSize wxDisplay::GetPPI() const
     return m_impl->GetPPI();
 }
 
+double wxDisplay::GetScaleFactor() const
+{
+    wxCHECK_MSG( IsOk(), 0, wxT("invalid wxDisplay object") );
+
+#ifdef wxHAVE_DPI_INDEPENDENT_PIXELS
+    // Use wxDisplayImpl::GetScaleFactor() directly, it should be implemented
+    // correctly for the platforms doing pixel scaling and using it simpler and
+    // more efficient than using GetPPI().
+    return m_impl->GetScaleFactor();
+#else
+    // Under the other platforms, we just compute the DPI ratio ourselves.
+    //
+    // We 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.
+    return m_impl->GetPPI().y / (double)GetStdPPIValue();
+#endif
+}
+
 int wxDisplay::GetDepth() const
 {
     wxCHECK_MSG( IsOk(), 0, wxT("invalid wxDisplay object") );