This avoids using an unbounded number of semaphores in between calls to
swapbuffers. Using two semaphores should be sufficient to synchronize
swaps.

In addition, fix tracking of VkPipelineStageFlags by creating a 2nd
vector parallel to the semaphores vector. The last fix assumed there
could only be 2 wait semaphores, but that bound only applied to signal
semaphores. After this change, there can only be one signal semaphore,
but there's still no bound to wait semaphores.

Bug: angleproject:3637
Change-Id: I7fbba67fa4bbdf62b9e9d530a924acd5236705d3
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/1688435
Commit-Queue: Michael Spang <spang@chromium.org>
Reviewed-by: Shahbaz Youssefi <syoussefi@chromium.org>