将附加/使用缓冲区的计数器值复制到另一个缓冲区。
Append/consume and counter buffers (see GraphicsBuffer.Target.Append, GraphicsBuffer.Target.Counter) keep track of the number of elements in them with a special counter variable. CopyCount takes a buffer as src, and copies its counter
value into dst buffer at given byte offset.
其最常与 Graphics.DrawProceduralIndirect 结合使用，
以渲染任意数量的图元，不必将它们的计数读回到 CPU。
On DX11 there is a restriction on the dst buffer - it must have been created with ComputeBufferType.Raw, GraphicsBuffer.Target.Raw, ComputeBufferType.IndirectArguments, or GraphicsBuffer.Target.IndirectArguments type. On other platforms dst can be any type.
另请参阅：SetCounterValue。