Add variable to control filter's out of bounds clipping

[Yanrishatum]

I would also add clipExtend, so it would still clip to reduce the workload (you usually don't want to overdraw parts user won't see), but will still provide a margin of overdraw if filter needs it. An example of where it's useful is drop-shadow filter, as it will have a cut shadow near screen edge opposite to shadow direction.

[ncannasse]

We discussed another approach for the feature that required this change.