Avoid warnings about mixing different enums with C++20

C++20 deprecates arithmetic operations with the elements of different
enums and at least clang 10 and MSVS 2019 already warn about this when
compiling in C++20 mode, so avoid such operations at least in the public
headers to avoid warnings in the applications using wxWidgets and C++20.

include/wx/defs.h

@@ -1352,7 +1352,7 @@ enum wxStretch
     wxGROW                    = 0x2000,
     wxEXPAND                  = wxGROW,
     wxSHAPED                  = 0x4000,
-    wxTILE                    = wxSHAPED | wxFIXED_MINSIZE,
+    wxTILE                    = 0xc000, /* wxSHAPED | wxFIXED_MINSIZE */
 
     /*  a mask to extract stretch from the combination of flags */
     wxSTRETCH_MASK            = 0x7000 /* sans wxTILE */

include/wx/pickerbase.h

@@ -147,7 +147,8 @@ protected:
     wxDEPRECATED_MSG("useless and will be removed in the future")
     int GetDefaultTextCtrlFlag() const
     {
-        return wxALIGN_CENTER_VERTICAL | wxRIGHT;
+        // Cast to avoid warnings about mixing elements of different enums.
+        return wxALIGN_CENTER_VERTICAL | static_cast<int>(wxRIGHT);
     }
 #endif // WXWIN_COMPATIBILITY_3_0