Like with CPU emulation, we generally don't want to fire off the threads
immediately after the relevant classes are initialized, we want to do
this after all necessary data is done loading first.
This splits the thread creation into its own interface member function
to allow controlling when these threads in particular get created.
Because of the recent separation of GPU functionality into sync/async
variants, we need to mark the destructor virtual to provide proper
destruction behavior, given we use the base class within the System
class.
Prior to this, it was undefined behavior whether or not the destructor
in the derived classes would ever execute.
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