Allow arbitrary extrapolation methods

Whenever a function needs to extrapolate values of non-existing pixels (i.e. pixels outside of the image), it should be possible to specify the extrapolation method to use.

The extrapolation methods I consider to be most useful are:

• replicate border
• replicate border, with alpha set to 0
• arbitrary constant
• wrap around
• arbitrary Python function

See the introduction to http://docs.opencv.org/modules/imgproc/doc/filtering.html for further explanation and other extrapolation methods. There are several Pillow functions where this comes in handy. For example,

image.transform(size, PIL.Image.AFFINE, data, PIL.Image.BICUBIC, extrapolate=REPLICATE_BORDER_ALPHA0)

might produce a rotated and scaled version of the source image on top of a transparent background so you can paste this transformed image into other images easily. Another example:

image.filter(filter, extrapolate=WRAP_AROUND)

might produce a blurred image that you can use for a tiled wallpaper without ugly tile borders.

If you need to prioritize, I consider Image.transform to be more important than Image.filter. (This is because it is quite easy to work around this issue for Image.filter as soon as this is implemented for Image.transform.)

Start by tracing the Image.transform entry point, which the issue identifies as the priority, and compare how Image.filter handles pixels outside the image. Define the supported extrapolation methods and verify that transform can apply them, including an arbitrary Python function, with tests covering the listed border behaviors and transparent output.