more const in format detection

BaseImporter::SearchFileHeaderForToken() expected a pointer to a non-const token list. This was probably an oversight, as nobody would realistically expect the function to change the list. Furthermore, it prevented token lists from being compiled to read-only memory, in some cases even causing the compiler to generate thread-safe initialization.

The list is now const and all callers declare their token lists static const, thus compiling them to read-only memory.

code/AssetLib/OFF/OFFLoader.cpp

@@ -94,7 +94,7 @@ bool OFFImporter::CanRead( const std::string& pFile, IOSystem* pIOHandler, bool
     else if (!extension.length() || checkSig)
     {
         if (!pIOHandler)return true;
-        const char* tokens[] = {"off"};
+        static const char * const tokens[] = {"off"};
         return SearchFileHeaderForToken(pIOHandler,pFile,tokens,1,3);
     }
     return false;