WaitGroup is a synchronization primitive that holds an internal counter that can incremented, decremented and waited on until it reaches 0.
WaitGroups can be used as a simple mechanism for waiting on a number of concurrently execute a number of tasks to complete.

Bug: b/139010488
Change-Id: I086859b81509076de3dbce8a5fde656ab4e4e347
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/34816Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Kokoro-Presubmit: kokoro <noreply+kokoro@google.com>