rebase to nano precision (requires Qt 4.8), thus fix conversion mess.
pro: most of the time i render < 0.66 ms =)

summarized

it's math.
fpsinterval is n*vblankTime so this moves it 1ms past the last assumed tick before the wanted interval spot, so that we don't have to fear to miss it.

Assuming we paint every 3rd frame:

A 1 2 3 4 5 - B 1 2 3 4 5 - C 1 2 3 4 5 - A
                              ^ here (C1)