fix game panel size on wxGTK #325

On wxWidgets 3.1.2, which is what vcpkg uses (and we use vcpkg for
Visual Studio) the wxEXPAND flag for the game area in the sizer throws
an XRC error dialog, removing the flag works correctly on Windows, but
not on wxGTK.

As a workaround, do not use the wxEXPAND flag on Windows.

TODO: with max zoom set, the game area is not centered vertically in
wxGTK.
Signed-off-by: Rafael Kitover <rkitover@gmail.com>

src/wx/panel.cpp

@@ -1034,7 +1034,16 @@ void GameArea::OnIdle(wxIdleEvent& event)
 
         // add spacers on top and bottom to center panel vertically
         GetSizer()->Add(0, 0, 1, wxEXPAND);
-        GetSizer()->Add(w,    0, gopts.retain_aspect ? (wxSHAPED | wxALIGN_CENTER) : wxEXPAND);
+
+        // On windows with the vcpkg version of wxWidgets which is 3.1.2, the
+        // wxEXPAND flag throws an XRC error, but everything works fine without it.
+        // On GTK however, the flag is necessary.
+#if defined(__WXMSW__)
+        GetSizer()->Add(w,    0, gopts.retain_aspect ? (wxSHAPED | wxALIGN_CENTER_HORIZONTAL) : wxEXPAND);
+#else
+        GetSizer()->Add(w,    0, gopts.retain_aspect ? (wxSHAPED | wxALIGN_CENTER_HORIZONTAL | wxEXPAND) : wxEXPAND);
+#endif
+
         GetSizer()->Add(0, 0, 1, wxEXPAND);
         Layout();