Sketch DM refactor.

dm/README

Index: dm/README
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-DM (Diamond Master, a.k.a Dungeon master, a.k.a GM 2).
-
-DM is like GM, but multithreaded.  It doesn't do everything GM does.
-
-DM's design is based around Tasks and a TaskRunner.
-
-A Task represents an independent unit of work that might fail.  We make a task
-for each GM/configuration pair we want to run.  Tasks can kick off new tasks
-themselves.  For example, a CpuTask can kick off a ReplayTask to make sure
-recording and playing back an SkPicture gives the same result as direct
-rendering.
-
-The TaskRunner runs all tasks on one of two threadpools, whose sizes are
-configurable by --cpuThreads and --gpuThreads.  Ideally we'd run these on a
-single threadpool but it can swamp the GPU if we shove too much work into it at
-once.  --cpuThreads defaults to the number of cores on the machine.
---gpuThreads defaults to 1, but you may find 2 or 4 runs a little faster.
-
-So the main flow of DM is:
-
-    for each GM:
-        for each configuration:
-            kick off a new task
-    < tasks run, maybe fail, and maybe kick off new tasks >
-    wait for all tasks to finish
-    report failures
-