Also return display DPI from wxScreenDC in wxGTK3

This is similar to the previous commit and is done for the same reasons:
screen DC needs to use the same DPI as the screen itself, instead of the
default Cairo 72 DPI.

src/gtk/dc.cpp

@@ -364,6 +364,12 @@ wxScreenDCImpl::wxScreenDCImpl(wxScreenDC* owner)
     gc->EnableOffset(m_contentScaleFactor <= 1);
     SetGraphicsContext(gc);
 }
+
+wxSize wxScreenDCImpl::GetPPI() const
+{
+    return wxGetDisplayPPI();
+}
+
 //-----------------------------------------------------------------------------
 
 wxMemoryDCImpl::wxMemoryDCImpl(wxMemoryDC* owner)