Blender Git Loki

Fix memory leak in point density

The issue was happening when having unconnected point density which
will cache data but will not free it because there's no actual call
to the actual sampling.

Now the idea is to make sure cache is zeroed on file load and undo
and then caching via RNA will free the data if any exists. This could
leave us with a single copy of cache in the node if it's not used,
but it's quite small amount of memory and it's not leaking.