seamlessClone takes time proportional to the size of the image (Bug #4445)

I've found that with a fixed hole size, when the size of the image increases the runtime increases substantially. For example, if I fill a 200x200 hole in a 500x500 image, it takes < 1 second. Then if I fill a 200x200 hole in a 3000x3000 image, it takes 20 seconds. The 500x500 was a cropped version of the 3000x3000 so that the hole and source could be identical for testing to make sure it wasn't a function of the data in the hole, etc.

My guess is that some processing is happening (derivatives being computed, etc.) over the whole image, even though they are only needed inside the source or hole region.