For completely unknown GL functions, return the original function

* CheckUnsupported handles all known compatibility GL functions and extension
  functions, so this would only be true for an unpublished or completely new
  extension. It might break, but if it's an internal function call it's better
  to pass it through.

renderdoc/driver/gl/gl_hooks_linux_shared.cpp

@@ -1417,8 +1417,10 @@ void *SharedLookupFuncPtr(const char *func, void *realFunc)
   CheckUnsupported();
 
   // for any other function, if it's not a core or extension function we know about,
-  // just return NULL
-  return NULL;
+  // return the real function pointer as this may be something internal
+  RDCLOG("Returning real pointer for entirely unknown function '%s': %p", func, realFunc);
+
+  return realFunc;
 }
 
 bool SharedPopulateHooks(bool dlsymFirst, void *(*lookupFunc)(const char *))