Blender Git Loki

Fix T35328: Disk caches of multiple particle systems on a single object overwrite each other

It was intended to work actually using particle cache's stack index
but this index might have been calculated incorrect in special case:

* With default cube scene, add particle system to the cube
* Add disk cache to the particle system
* Save file and reload it
* Add another particle system and enable disk cache

This would lead to two point caches with the same stack index of zero.
This happened because point cache indices list wasn't stored in the
.blend file so once you've reload your file blender doesn't know anything
about number or point caches used.

And what was even more confusing is that point cache indices list was
trying to be load from the file, but this failed because it wasn't in the
file.

This commit solves the root of the issue which is ability of producing
.blend file with two point caches using the same disk cache. This is
done by making it sure that point cache indices list is stored in the
.blend file. And also made it so disabling disk cache will tag it to
recalculate stack index.

Old broken files wouldn't magically start working, but fixing them is
rather simple manually by toggling Disk Cache option.

Reviewers: lukastoenne, brecht

CC: sergof

Differential Revision: https://developer.blender.org/D286