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/Common/BaseImporter.cpp

@@ -155,7 +155,7 @@ void BaseImporter::GetExtensionList(std::set<std::string> &extensions) {
 // ------------------------------------------------------------------------------------------------
 /*static*/ bool BaseImporter::SearchFileHeaderForToken(IOSystem *pIOHandler,
         const std::string &pFile,
-        const char **tokens,
+        const char * const *tokens,
         unsigned int numTokens,
         unsigned int searchBytes /* = 200 */,
         bool tokensSol /* false */,