Blender Git Loki

Major optimization for cache size when storing child strands.

The basis for this optimization is that child strands have a shape that
does not actually change over time: the particle code does not enforce
this formally, but in fact all the clump/kink/roughness algorithms take
great care to not introduce changes of shape over time.

The final look of child hairs is achieved only by following the parent
hair deformation, i.e. by applying the offset of the matching parametric
position on the parent strand.

This means that for caching child strands it is sufficient to store the
full vertex data only in the first sample, and then apply the parent
deformation on subsequent frames using only the animated root matrices.

In test cases this seems to reduce the cache size to something between
10-20 % of the original size (down to some hundred MB from some 10 GB).
Further optimizations could be achieved by using smaller data types,
such as quaternions instead of 3x3 or 4x4 matrices for the remaining
child data.