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/Irr/IRRMeshLoader.cpp

@@ -101,7 +101,7 @@ bool IRRMeshImporter::CanRead(const std::string &pFile, IOSystem *pIOHandler, bo
          *  must return true here.
          */
 		if (!pIOHandler) return true;
-		const char *tokens[] = { "irrmesh" };
+		static const char * const tokens[] = { "irrmesh" };
 		return SearchFileHeaderForToken(pIOHandler, pFile, tokens, 1);
 	}
 	return false;