First, is it possible to write incremental loader that does *not* read
the entire input file? This is something that would speed up my loader
a lot. PSD files contain a preview near the begining of the file
followed by a lot of other stuff, like layers, masks and so on. The
loader reads only the preview and the rest is not needed, but often
the rest takes a lot of space. One possible way to stop reading is to
return FALSE from load_increment function, but this is considered as
error indication. Is there any way to work around this behaviour?