Emit multiple data chunks from output Streams (was: Proper streaming)

It would be very cool if sharp could support proper streaming. As it is now, it will always emit exactly one data event which is the whole image. It kind of defeats the purposes of streams.

As I see it there is two ways to implement this.

1. The hard, but robust way.

   Every time that libvips have data that should be written, it should call a function that we have defined. We then use this data to generate a data-event on the stream. It would be really cool if libvips could provide a state-machine that has a synchronous function that gives the next chunk. That way pause and unpause would work perfectly.

2. The quick and easy fix.

   We can also create a named pipe and tell libvips to write it to that file via vips_image_write_to_file. To the user we would then return a ReadStream as returned by fs.createReadStream(path).

Start by reviewing the output Stream implementation and how libvips currently writes the complete image. Compare the proposed callback/state-machine approach with using a named pipe and fs.createReadStream(path). Done means output emits multiple data events while preserving usable pause and unpause behavior.