Border effect should respect PNG transparency / image shape

[JordanMagnuson]

Thank you for implementing border as an effect! This border implementation is much preferable to the old one.

One problem remains: the border effect does not respect PNG transparency: it only creates a rectangular border around any image, regardless of the image shape. It should work like the drop shadow effect, which respects PNG transparency, and creates a shadow around the particular shape of the image.

See this screenshot comparing miniPaint border effect to Photoshop stroke effect, for the same image: https://i.imgur.com/5qzt1Ml.jpeg