Re: Atomic renderers

[Chris Warburton]

We only use `render_page` in one place, with a `DEST` argument which is
the output filename. That gets written to, read back, etc. in a highly
non-atomic way.

I've now changed that so we give a temp filename as `DEST`, then we
check if it's empty (error if so), and finally we move it (presumably)
atomically into the place of `$out`.