Blender Git Loki

Fix T57760: Depsgraph 'object_instances' showing incorrect list of objects.

It appears that Python gets next item before using current one, which
would break our Depsgraph instance iterator (since only current item is
valid there, we use the same memory at each iteration).

Working around that with an ugly ping-pong game between two sets of
iterator data, so that previous one (C RNA-iterator-wise) remains valid
memory for Python to access to.