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.
It replaces current fuzz setup with 5 new fuzzers integrating them with
top-level Makefile. It now supports multiple fuzzing engines and Google's
oss-fuzz. By default, the fuzzers are linked against an standalone runner to
easily reproduce bugs.
