| src | Append/consume buffer to copy the counter from. |
| dst | A buffer to copy the counter to. |
| dstOffset | Target byte offset in dst. |
Copy counter value of append/consume buffer into another buffer.
Append/consume buffers keep track of the number of elements in them with a special counter variable. CopyCount takes an append/consume buffer as src, and copies its counter
value into dst buffer at given byte offset.
This is most commonly used in conjunction with Graphics.DrawProceduralIndirect,
to render arbitrary number of primitives without reading their count back to the CPU.