A semaphore pool is implemented to allow dynamic allocation of
semaphores as needed when breaking up a frame with flushes.  The pool is
used both for acquiring the next image and for chaining mid-frame
submissions.

RendererVk::flush() is changed so that instead of taking the wait/signal
semaphores as parameters, it would use the last known signaled semaphore
as wait semaphore and allocates a semaphore for signaling.  It would
additionally wait for any extra semaphore provided externally (i.e. the
surface's image acquire semaphore).

Bug: angleproject:2504
Change-Id: Iecd2d5535230c48b26a6b7d078710af8730121da
Reviewed-on: https://chromium-review.googlesource.com/c/1276805
Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>