Make engine subprocess-based
Make engine subprocess-based

The bindings I was using require the latest version of VIPS, which
is different from the one we have in production. Those bindings
are particularly annoying to install, too.

This simplifies the engine, making it more like the other ones,
caring only about converting the image to a supported format.

Bug: T119184
Change-Id: I51502cd0bee47691ca0b40cfd6a43eb9d73a4ef5

wikimedia/thumbor-vips-engine