Particles and Particles2D preprocess property doesn't do anything

[Keetz]

Operating system or device, Godot version, GPU Model and driver (if graphics related):
Godot 2.1.4 stable

Issue description:
Original issue #12460 has been fixed on master, but this is still a problem in 2.1.4, so this is basically just a copy of the original issue.

Original issue description:

I added a Particle node to the scene and I set the particle lifetime to 5, and the preprocess time to 5. But when I play the scene the particle system start from the beginning, and doesn't have 5 seconds worth of preprocessing. This happens to both 3D and 2D particles.

#13277 is the PR that fixes it on master.

[akien-mga]

From a quick glance it doesn't seem like a fix similar to #13277 would be applicable.

The logic seems correct a priori, so one needs to dig deeper:
https://github.com/godotengine/godot/blob/2.1/scene/2d/particles_2d.cpp#L462-L466

I don't think 3D Particles have pre-process in 2.1 though?

[akien-mga]

The 2.1 branch is no longer actively worked on by engine developers, unless critical fixes are needed for current games in production. As such we are now closing non-critical bug reports affecting only the 2.1 branch.

Please comment if this was incorrectly triaged and is still relevant in 3.x versions.