Make the semaphore pool larger, such that there are enough for any number of swapchain images AND frames in flight.

[mcourteaux]

Constructed this together with @pezcode.

This very simple change fixes the issue on my device.

Summary:

• my device made a swapchain of 2 images.
• bgfx has by default a max frame latency of 3; so 3 frames in flight.
• there is therefore missing one semaphore to have enough for the 3 frames in flight.

This fixes #3302 and replaces #3303.

[bkaradzic]

It doesn't look like fix. There should be association between semaphore that's queued and semaphore that's about to be used. And then before passing semaphore to vkAcquireNextImageKHR it should be waited for queue that would wait for this semaphore. This wait would be noop in most of cases since there is enough semaphores, but it would be proper synchronization for those cases where queue is not finished.