Convert `Picture` to `BufferedImage`

[noelwelsh]

Create a stable Java2D specific method to convert a Picture to a BufferedImage. This will allow fun pixel art in conjunction with #85. This should be a new kind of effect type class, I think.

We already have a ToPicture type class. Perhaps FromPicture is the natural counterpart here? Not sure they would be symmetric though---rendering a picture is an effect but converting to a picture is not.

[fireddd]

Hi Noel, Is this what is needed here?? https://stackoverflow.com/questions/13605248/java-converting-image-to-bufferedimage

[noelwelsh]

That's not really the issue. This is about having a way to convert the Picture type in Doodle into a BufferedImage. All the code already exists, it just needs to presented in a stable API.