Refactoring to refamiliarize myself with SkTaskGroup and SkThreadPool.
This adds an SkExecutor interface to describe how we use SkThreadPool,
with a global setter and getter for a default instance. Then I rewrote
SkTaskGroup to work with any executor, the global default by default.
I also think I've made the SkTaskGroup::wait() borrow logic clearer
with the addition of SkSemaphore::try_wait(). This lets me keep the
semaphore count and actual count of work in perfect sync.
Change-Id: I6bbdfaeb0e2c3a43daff6192d34bc4a3f7210178
Reviewed-on: https://skia-review.googlesource.com/8836
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
