modules/voronoi: spot-fix a crash Phil reported

The repro is:
--seed=0x64f6820b '--module=compose,layers=blank\,pixbounce\\\,pixmap_size\\\=0.8\\\,pixmap\\\=err\,pixbounce\\\,pixmap_size\\\=0.4\\\,pixmap\\\=ignignokt,texture=voronoi\,cells\=512\,randomize\=on' '--video=mem,size=3840x2160'

The major culprit seems to be the combination of high resolution,
and small number of voronoi cells (cells=512), with randomize=on
which exercises jumpfill every frame.

The way jumpfill is implemented currently is racy by design to
allow threading, and mostly works fine despite not really being
how the algorithm is intended to work.

The assumption has been, something like:
"the seeds are already placed before the threaded phase, so the
threaded jumpfill should at least find stable seed cells in the
face of racing against other tiles being jumpfilled
simultaneously"

But it appears that assumption isn't always true, in that we
won't necessarily find one of the seed cells at the start of the
jumpfill when there aren't that many cells (512) compared to the
area of the voronoi (3840x2160).

By noticing when we've finished a tile's jumpfill with remaining
unassigned cells, we can just repeat the jumpfill, with time
passed, and the other tiles will have made progress on their work
propagating more knowledge of where cells are... so the
subsequent pass will probably leave nothing unassigned.

This approach sucks, but stops the crashing.

It'd also be possible to just change the way cells are looked up
so there's no potential for a NULL pointer dereference, just have
some uninitialized cell color which gets shown erroneously in the
output.  That avoids the computational cost of repeating the
tile's jumpfill, and likely nobody would notice the likely single
pixel of error for a single frame.

I'm just doing this quick and dirty fix to prevent the crashing
for now, and would like to just revisit voronoi more thoroughly
with an eye towards decoupling the voronoi cost from the
resolution.  It's a cheap hack the way there's a distance entry
per pixel, done just to simplify the implementation when I
slapped it together on a Zephyr train ride.

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