A shared threadpool, used to draw widgets concurrently.
NEW: A widget can opt to be drawn alone, and use the threadpool for its own drawing itself.
Can ONLY be called from onDrawRaw AND when the flag flagDrawAloneRaw is used,
or from onDrawPBR AND when the flag flagDrawAlonePBR is used.
A shared threadpool, used to draw widgets concurrently. NEW: A widget can opt to be drawn alone, and use the threadpool for its own drawing itself. Can ONLY be called from onDrawRaw AND when the flag flagDrawAloneRaw is used, or from onDrawPBR AND when the flag flagDrawAlonePBR is used.