This should give us the ability to send off more than one frame to
the presentation engine at once. Instead of using a single pair of
Semaphores to lock each surface, we make a Semaphore pair per
Swapchain image.

BUG=angleproject:1898

Change-Id: I9e833ed9969a79617d0a8968b0d5a25c27139e87
Reviewed-on: https://chromium-review.googlesource.com/672149
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Frank Henigman <fjhenigman@chromium.org>