Fixed GPUParticles2D preprocessing bug

[SoulForMachine]

When awaiting on timer before switching to a scene containing the particles, GPUParticles2D is first preprocessed with an initial position, and then updated to the set position on the next frame. Details of the fix in my comment on the bug page.

Fixes #75723

[akien-mga]

I haven't reviewed in depth, but I'm pretty sure adding a flush here is not going to be an acceptable hack, it has too many implications. So adding a change request pre-emptively to block merging, but this can still be discussed and reviewed further as a good starting point for the proper fix.

[SoulForMachine]

I haven't reviewed in depth, but I'm pretty sure adding a flush here is not going to be an acceptable hack, it has too many implications. So adding a change request pre-emptively to block merging, but this can still be discussed and reviewed further as a good starting point for the proper fix.

Yes, I understand that this will have to be fixed properly at some point, but as I said in my comment on the bug page, this same function has this same hack in 2 other places already (message queue flush before xform notification flush), so I thought that we might want to do this for now so that particles preprocessing works fine in this scenario.

I'm still pretty new to the project, so I don't know at this point what the proper solution might be. The other two hacks were done by @reduz and I'm guessing he had a reason to leave it like that for quite a while. Maybe he could comment on this pr.

[akien-mga]

Out of curiosity, I checked whether this would solve the seemingly related 3D equivalent of this bug #71935, but it doesn't seem to.