This progress report was utterly broken and useless when not exporting
animations, and not great either with animation export. This commit
fixes:
* Not generating any progress report when exporting a single frame.
* Assuming actual data export covers 100% of progress.
That last point is the biggest change, now actual export (be it
animated or still) only covers 75% of the whole range. The rest is used
to cover depsgraph building and updates (which can be noticiably slow
with big scenes), stage writing, etc.
Progression is still far from linear in time, but at least it gives
better feedback on actual progress of the whole export.
