Don't (ab)use vk::Fence for its start() finish() events.
sw::WaitGroup shares the same interface, so use that.

Allows vk::Fence to have its destructor deleted again.

Bug: b/133127573
Change-Id: I747ea6965bf85343c720235083b309d8b855a950
Reviewed-on: https://swiftshader-review.googlesource.com/c/SwiftShader/+/31684Tested-by: Ben Clayton <bclayton@google.com>
Reviewed-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>