make the buffer recycle list per image

so now recycle lists are short, scale with pipeline complexity, and
buffers are always appropriately sized for the image instead of being
slowly sized up to the max size for the pipeline

before:

$ vips sharpen k2.jpg x.jpg --radius 20
memory: high-water mark 38.99 MB

after:

$ vips sharpen k2.jpg x.jpg --radius 20
memory: high-water mark 29.46 MB