pyvips: Memory leak when saving pyramid files

When I open a TIFF image using access='sequential' and then save it using compression='jpeg' memory is not released.

Minimal example to reproduce the issue (run passing a large image file path as first parameter:

#!/usr/bin/env python3

import sys

import pyvips

fpath = sys.argv[1]

i = 1
while True:
    print('Iteration {}'.format(i))
    with open(fpath, 'rb') as fh:
        img = pyvips.Image.new_from_buffer(fh.read(), '', access='sequential')
        img.tiffsave('/tmp/flush.tif', compression='jpeg', Q=90)
    i += 1

You can observe memory constantly increasing until eventually the program crashes.

Note that if I don’t use sequential access, or JPEG compression, the memory is flushed as normal.