Add synchronization primitive to indicate to the app when buffer is complete

[russell-taylor]

For cases where we don't finish copying or rendering and displaying the frame before returning to the caller, provide a sync primitive so that the caller can tell when it is safe to re-fill a buffer because it is no longer being used for rendering. One place to check is asynchronous time warp, where the image is maintained for awhile.

This could be a function that they can call rather than a primitive, enabling it to be generic. The function should probably take a parameter indicating which frame it is for.