nx2: present no longer waits for its own frame; cancelled slot events signal

- vi2 229 now waits for the previous frame's fence by default, so the
  game prepares frame N+1 while the GPU thread runs frame N. Waiting for
  the current frame serialized them: 8-56 ms of each 34-105 ms frame was
  spent blocked. Title 1 now blocks under 1 ms per frame on average and
  its GPU thread runs ~90% busy, which is the next limit.
  EDEN_NX2_FRAMES_AHEAD=0 restores the old wait.
- syncpt 202 (cancel a slot request) signals the slot's event when the
  request was still pending. NvOsEventPool waits on it again after its
  ~100 ms wait times out; title 1 hung there forever whenever a mid-frame
  wait outlasted 100 ms with the GPU thread a frame behind (2 of 4
  runs). Two 185 s runs with 2-3 cancels each completed.
- EDEN_NX2_VSYNC_HZ=<rate> sets the vi2 vsync event rate (experiment;
  240 Hz did not shorten title 1's frames).

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