Callback to actually perform the data filtering

Specified as filter.apply, this is the callback that actually filters data. If it successfully writes the output, it should return 0. Like check, it can return GIT_PASSTHROUGH to indicate that the filter doesn't want to run. Other error codes will stop filter processing and return to the caller.

The payload value will refer to any payload that was set by the check callback. It may be read from or written to as needed.