Fix renderTarget crash when using Lanczos resampling filter.

The Lanczos filter doesn't do its own work to create the renderTarget so it doesn't need to prevent the super from doing so.

This is the crash described in #389. I guess that this isn't the only place that something like this goes wrong. I submit this pull request to try to help. If it is on the right track I'm happy to do the grunt work spreading the implementation elsewhere.