Hack to make wxHtmlPrintout unit test pass under wxGTK3

Set the "printer" PPI explicitly for wxMemoryDC used in the test to
ensure that it's the same in all ports: currently wxGTK3 stands out
because it uses 72 DPI unlike wxMSW and wxGTK2, which use 96.

tests/html/htmprint.cpp

@@ -70,6 +70,15 @@ TEST_CASE("wxHtmlPrintout::Pagination", "[html][print]")
     const wxFont fontFixedPixelSize(wxFontInfo(wxSize(10, 16)));
     pr.SetStandardFonts(fontFixedPixelSize.GetPointSize(), "Helvetica");
 
+    // We currently have to do this with wxGTK3 which uses 72 DPI for its
+    // wxMemoryDC, resulting in 3/4 scaling (because screen DPI is hardcoded as
+    // 96 in src/html/htmprint.cpp), when rendering onto it. This makes the
+    // tests pass, but really shouldn't be necessary. Unfortunately it's not
+    // clear where and how should this be fixed.
+#ifdef __WXGTK3__
+    pr.SetPPIPrinter(wxSize(96, 96));
+#endif
+
     wxBitmap bmp(1000, 1000);
     wxMemoryDC dc(bmp);
     pr.SetUp(dc);