GitLab

DM writes out its images in a hierarchy that's a little different than GM,
so this can't read GM's output.  But it can read its own, written with -w.

Example usage:
  $ out/Release/dm -w /tmp/baseline
  $ out/Release/dm -r /tmp/baseline -w /tmp/new
  (and optionally)
  $ mkdir /tmp/diff; out/Release/skdiff /tmp/baseline /tmp/new /tmp/diff

GM's IndividualImageExpectationsSource and Expectations are a little too eager
about decoding and hashing the expected images, so I took the opportunity to
add DM::Expectations that mostly replaces skiagm::ExpectationsSource and
skiagm::Expectations in DM.  It mainly exists to move the image decoding and
comparison off the main thread, which would otherwise be a major speed
bottleneck.

I tried to use skiagm code where possible.  One notable place where I differed
is in this new feature.  When -r is a directory of images, DM does no hashing.
It considerably faster to read the expected file into an ...