Frees as many buffers from the pool as possible.
- iOS 9.0+
- macOS 10.11+
- tvOS 9.0+
- watchOS 4.0+
- Core Video
CVPixelto be flushed.
excessto free all unused buffers regardless of their age.
By default, this function will free all aged out buffers. Setting the
k flag will cause this call to free all unused buffers regardless of age.