Fix paper size correctly for printouts under MSW and Mac

This was broken by 048b7f44ec which wasn't
supposed to change anything, but did because it effectively replaced the
call to wxDC::GetPaperRect() with wxDC::GetSize() when initializing
wxPrintout::m_paperRectPixels, and while GetPaperRect() is the same as
GetSize() in wxGTK, it returns the rectangle including non-printable
margins in wxMSW and wxOSX, and must be used here instead.

Closes #18565.

Closes https://github.com/wxWidgets/wxWidgets/pull/1640

src/common/prntbase.cpp

@@ -640,6 +640,8 @@ void wxPrintout::GetPageInfo(int *minPage, int *maxPage, int *fromPage, int *toP
 
 bool wxPrintout::SetUp(wxDC& dc)
 {
+    wxCHECK_MSG( dc.IsOk(), false, "should have a valid DC to set up" );
+
     SetPPIScreen(wxGetDisplayPPI());
 
     // We need to know printer PPI. In most ports, this can be retrieved from
@@ -659,7 +661,10 @@ bool wxPrintout::SetUp(wxDC& dc)
     SetDC(&dc);
 
     dc.GetSize(&m_pageWidthPixels, &m_pageHeightPixels);
-    m_paperRectPixels = wxRect(0, 0, m_pageWidthPixels, m_pageHeightPixels);
+    // This is ugly, but wxDCImpl itself has GetPaperRect() method while wxDC
+    // doesn't (only wxPrinterDC does), so use GetImpl() to avoid having to use
+    // a dynamic cast.
+    m_paperRectPixels = dc.GetImpl()->GetPaperRect();
     dc.GetSizeMM(&m_pageWidthMM, &m_pageHeightMM);
 
     return true;