- Fix failure building on Xcode 14.
- Track frame interval statistics, regardless of whether performance
tracking is enabled.
- Determine wait time for swapchain presentations from frame intervals.
- MVKSwapchain call markFrameInterval() from within mutex lock.
- MVKDevice rename addActivityPerformance() to addPerformanceInterval()
and addActivityByteCount() to addPerformanceByteCount().
- Add documentation about performance being measured in milliseconds.
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