RoundedCorners and Circle image processor Border width

[ejensen]

The RoundedCorners image processor has an existing Border parameter. However It was not respecting the border.width when drawing the stroke. The stoke was always drawn with a width of 1.
This PR corrects the width of the stroke border in the RoundedCorners processor.

This PR also adds an optional border parameter to the Circle processor. The new parameter defaults to nil in order to prevent a breaking API change.

[kean]

Thanks, @ejensen! This looks good. I don't remember why Border was implemented this way initially, it's great that it now works as expected.

I'm going to merge this and I will also add a typealias ImageProcessor. RoundedCorners.Border = ImageProcessor.Border to make sure there are no breaking API changes. Thanks again. If you see any other issues in processors, please let me know. They are a new addition in Nuke 8, not as polished yet.