Pass known page breaks as const ref to wxHtmlDCRenderer::Render()

This method doesn't modify the known page breaks, it only uses them to
find the next one, so make it more clear by using const reference in the
function signature.

src/html/htmprint.cpp

@@ -141,7 +141,7 @@ void wxHtmlDCRenderer::SetStandardFonts(int size,
 }
 
 int wxHtmlDCRenderer::Render(int x, int y,
-                             wxArrayInt& known_pagebreaks,
+                             const wxArrayInt& known_pagebreaks,
                              int from, int dont_render, int to)
 {
     wxCHECK_MSG( m_Cells, 0, "SetHtmlText() must be called before Render()" );