Fix sub_emitter does not process during request_particles_process

[xuhuisheng]

• Fixes: GPUParticles2D.request_particles_process does not process sub_emitters #111885

Issue Description

This issue caused by processing order. When we config a sub_emitter, we want the sub_emitter will process just after every parent step.

But when we use request_particles_process to skip a short of time, ParticlesStorage process first parent before sub_emitter. That means if we want to delay 1 second before render particles, ParticlesStorage first let parent particles process 1 seconds, then handle the sub_emitter, so sub_emitter always start from parent particles ended. The sub_emitter can only be emitted from the parent's last position of request_particles_process time.

• fixed_fps: 30 (30 frame per seconds)
• request_particles_process(1.0) (process particles 1 second before start)
• first, let parent particles process 30 frames
• second, let sub_emitter particles process 30 frames.

What we want is processing parent and sub_emitter one by one during the request_particles_process time.

Solution

After the '_particles_process' is executed, if there is a sub_emtter, it will continue to execute the sub_emitter and save it in the processed_particles. If it is found that the particles have already been processed, it will jump out to avoid duplicate processing.

particles.webm

Etc

I am not sure the processed_particles.has(particles->particle_buffer) is suitable for HashSet matching. Any advice?

[xuhuisheng]

After detailed testing, the particles system is more complicated than I thought. Current PR maybe result as a duplicated processing to looks like correct.

Marked this PR as draft, until confirmed new behaviors equals as before.