Many codecs can take a huge amount of time attempting to read large
random objects. jpeg_read_header(), for example, can take ~10s on a 1mb
of random data.
Ignore fuzz objects over 100kb.
See https://bugs.chromium.org/p/oss-fuzz/issues/detail?id=24383
oss-fuzz normally fuzzes with memory buffers, but a lot of the libvips
loadres, especially CSV and PPM, only work from the filesystem.
This fuzzer writes the oss-fuzz buffer to the FS, then fuzzes that.