The current behaviour (close input handles on unref) works for languages
like C / C++ / Python / Rust / etc. where things get unreffed automatically
when they go out of scope.
On languages like Ruby / C# / node / etc. where things are unreffed on GC,
files can stay open for a long time after you've finished with them. This
interacts in an unfortunate way with the Windows default of refusing to
remove open files.
This change closes file handles as soon as the scan of the input file
finishes, and therefore produces something closer to expected behaviour
for GCd languages on Windows.
see https://github.com/kleisauke/net-vips/issues/12
