So now cases when object has both hair motion blur and deformation motion blur
vector pass is all correct.
We could get rid of the flag in the future, still need to look deeper into all
the areas trying to find a more clear solution.
Issue was caused by mismatch in pre/post transform matrix spaces for mesh and
curve vectors. This happened because of current way how static transform apply
works: it only stores post/pre in the world space if there's triangle motion
exists. This lead to situation when there's no triangle motion happening but
was hair motion happening.
After long time of trying to solve it in a nice way, ended up solving it in
a bit slow way -- pre/post transform is still storing in the same spaces as
they used to be stored and just convert hair pre/post position to a world
space in the kernel.
This is because currently it's not so clear how to deal with cases when curve
and mesh motion needs different space of pre/post transform (which happens in
cases when only one of the motions exists).
Would think of some magic, and meanwhile artists could be happy with proper
render results.
These are internally stored as a 3D image textures, but accessible like e.g.
UV coordinates though the attribute node and getattribute().
This is convenient for rendering e.g. smoke objects where data like density is
really a property of the mesh, and it avoids having to specify the smoke object
in a texture node, instead the material will work with any smoke domain.
