Add Encoder fuzzers

This also includes a helper "fuzzer" for making a corpus.
Point it at an image or folder of images and it will
decode those images and write the SkPixmap's bytes to
disk, such that the fuzzer will be able to read in
those bytes as if it had decoded the image (or gotten
it from another source).

Bug: skia:
Change-Id: Iaf223a39078f2b62908fb47929add5d63f22d973
Reviewed-on: https://skia-review.googlesource.com/117367
Reviewed-by: Leon Scroggins <scroggo@google.com>
Commit-Queue: Kevin Lubick <kjlubick@google.com>
6 files changed