Blender Git Loki

Fix ugly recursive pattern in RNA PointCache data layout.

PointCache was having a collection of items of PointCache type, having a
collection of items of PointCache type, having...

Nuff said.

For now, chose the 'ugly' way to fix it, that is, the one that changes
nothing to API and scripts using it: we define another 'PointCacheItem'
RNA type for items of our point cache collection, which has exact same
interface as PointCache except for the collection.

This is doomed to be rewritten at some point anyway, not worth spending
time trying to define a really correct data layout for now.