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- Various legacy game thread code assumes the render thread will never be more than one frame behind. The original change in 36468180 switched from syncing the GT with the RT, to syncing the GT with the RHIT. That left the render thread "floating" in the center of the pipeline, and led to cases where resources are deleted too soon. - New approach is to always sync with the GT with the N-1 RT frame, so the GT is never too far ahead of the RT. This maintains compatibility with the legacy GT code paths. In addition to the GT->RT sync, we also sync with the RHIT to prevent the engine running ahead, which was the original bug that 36468180 was fixing. - "r.GTSyncType" mode 0 now allows for 1 frame of GT->RT overlap, and 2 frames of GT->RHIT overlap. - For debugging purposes, "r.GTSyncType" can also be made negative, which increases the number of GT->RHIT overlap frames, e.g. "r.GTSyncType -3" gives 5 frames of GT->RHIT overlap. While this is not overly useful in a shipped title, it can be used to prove the correctness of the rendering pipeline. - Merged the FDeferredCleanupInterface / FPendingCleanupObjects processing into the FFrameEndSync code path, plus made FFrameEndSync a static singleton. This allows us to manage the N frames of overlap between the GT and RHIT in a central place, and correctly cleanup deferred resources when the RT fences have passed. In future, we will need to revisit this as part of the frame pacing initiative. Game thread code should be written to not require render thread fences for correctness / threadsafety. #jira UE-223692 #rb zach.bethel [CL 36758377 by luke thatcher in 5.5 branch]