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Fix T38811: Cycles particle ids are inconsistent when using multiple particle systems.

Problem is that the particle systems in the cycles database are not
stored in a well-defined order. This means the particle_id for dupli
objects can not simply be assigned using a global running index during
sync.

Now the particle index is assigned locally for each particle system.
When transferring particle data to the device as a single texture, the
particle indices are offset based on the final order of particle systems
in the database.

Reviewers: brecht

Reviewed By: brecht

CC: Andreas80

Differential Revision: https://developer.blender.org/D352