main: prevent ticks from going backwards

Outside of overflow (which I'm ignoring for now) ticks shouldn't
go backwards.  With the introduction of adding the frame-buffer
delays, which vary, there's the potential for the delay to go
from large to short in quick succession, to such a degree that
the next render's now+delay is in the past relative to the
previous now+delay.

For now this simple fix is to just track the last_ticks and
always use the maximum of the last_ticks and now+delay, ensuring
it never goes backwards.

This was making alphazed exit prematurely at spurious times by
sending the rocket_row into oblivion because (ticks - last_ticks)
was negative w/unsigned arithmetic.  This will all get more work,
and maybe ticks should be allowed to go backwards actually, but
some things are assuming that's not the case as-is.

Regardless, it's not desirable for ticks to go backwards because
of the frame-buffer delay.  In that case just chill on the ticks
advancement for a frame.  This will need revisiting for sure, as
I don't think re-rendering the exact same tick as the last frame
is likely to be what's wanted either.  Probably some little
advancement should still be performed...

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