vgsp: xw-mirror writes one learned frame-step ahead of the counter

r866: the mirror tracked perfectly yet every wait still paid the driver's
~3 s re-check sleep (one '(0)' success print every 20-80 s; the GL client
hung minutes on its X socket). Cause: the driver increments its counter
and then enters the wait; a member that only tracks the counter always
loses that entry-check race (mirror wakes <=3 ms after the change; the
driver probes within microseconds), so the check fails and the driver
sleeps out its retry interval. Writing member = counter + last-observed-
delta (learned from each counter change, 0 until the first) makes the
driver's own increment land exactly on the pre-written member value, so
the first probe after submission succeeds and frames complete at the
driver's pace.

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