Add wxPrintout::SetUp() to reuse the same code in all ports

MSW, GTK, OSX and Postscript implementations all did almost exactly the
same thing to initialize wxPrintout, so extract this common code into a
new wxPrintout method and just call it instead.

There should be no changes in behaviour.

src/common/prntbase.cpp

@@ -638,6 +638,33 @@ void wxPrintout::GetPageInfo(int *minPage, int *maxPage, int *fromPage, int *toP
     *toPage = 1;
 }
 
+bool wxPrintout::SetUp(wxDC& dc)
+{
+    SetPPIScreen(wxGetDisplayPPI());
+
+    // We need to know printer PPI. In most ports, this can be retrieved from
+    // the printer DC, but in others it is computed (probably for legacy
+    // reasons) outside of wxDC code, so don't override it if it had been
+    // already set.
+    if ( !m_PPIPrinterX || !m_PPIPrinterY )
+    {
+        SetPPIPrinter(dc.GetPPI());
+        if ( !m_PPIPrinterX || !m_PPIPrinterY )
+        {
+            // But if we couldn't get it in any way, we can't continue.
+            return false;
+        }
+    }
+
+    SetDC(&dc);
+
+    dc.GetSize(&m_pageWidthPixels, &m_pageHeightPixels);
+    m_paperRectPixels = wxRect(0, 0, m_pageWidthPixels, m_pageHeightPixels);
+    dc.GetSizeMM(&m_pageWidthMM, &m_pageHeightMM);
+
+    return true;
+}
+
 void wxPrintout::FitThisSizeToPaper(const wxSize& imageSize)
 {
     // Set the DC scale and origin so that the given image size fits within the