Add tests for `BufferedImageWriter`

[noelwelsh]

There are several ways this can be tested:

• convert to BufferedImage and then save should be the same as writing directly to a file
• convert to BufferedImage, load as bitmap, and then convert to BufferedImage again should create the same image (make sure the Frame margin is 0)
• using the existing golden testing infrastructure, convert to BufferedImage, save, and check it's the same as a previously created image.