DM: Add support for reading a directory of images with --expectations (-r).

Add support for reading a directory of images with --expectations (-r).

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 SkBitmap and do a
byte-for-byte comparison than to hash the two bitmaps and check those.

The example usage above isn't quite working 100% yet.  Expectations on some GMs
fail, even with no binary change.  I haven't pinned down whether this is due to
  - a bug in DM
  - flaky GMs
  - unthreadsafe GMs
  - flaky image decoding
  - unthreadsafe image decoding
  - something else
but I intend to.  Leon, Derek and I have suspected PNG decoding isn't
threadsafe, but are as yet unable to prove it.

I also seem to be able to cause malloc to fail on my laptop if I run too many
configs at once, though I never seem to be using more than ~1G of RAM.  Will
track that down too.

BUG=

Committed: http://code.google.com/p/skia/source/detail?r=12596

[mtklein, 2013-12-06 21:42:13]

On 2013/12/06 21:37:13, mtklein wrote:

Forgot to mention: on my laptop checking against images with --expectations adds
an extra 6% wall time in Release (8.6s -> 9.1s) and has no effect on Debug (56s
-> 56s).  A Z620 will probably be affected even less, in both modes.

[bsalomon, 2013-12-09 16:00:21]

lgtm

[commit-bot: I haz the power, 2013-12-10 14:45:22]

CQ is trying da patch. Follow status at
https://skia-tree-status.appspot.com/cq/mtklein@google.com/108963002/20001

[commit-bot: I haz the power, 2013-12-10 14:53:25]

Change committed as 12596