Fix collided 3D GPU particles sometimes jittering

[Rudolph-B]

Fixes an issue found by @RPicster in #87320.

What would happen is a particle will collide with a collider, the collision system would detect it and attempt to move the particle out of the collider. Sometime this would result in a particle that still touches the collider and other times not (When it touched was cause by a floating point precision error). If the particle did not touch the collider it would start moving again even though visually it is touching the collider. This causes "collided" particles to jitter. It is much more visible at lower Particle "Fixed FPS"

The solution I found was to introduce an EPSILON term in collision calculations. This ensures that collided particle will always "touch" the collider after the particle has been moved. This solution was applied to the Forward+ renderer as well as the OpenGL renderer.

An MRP showcasing the issue and solution for multiple collider types

[clayjohn]

Tested locally on the RD and GLES3 backends and can confirm that this PR fixes the jittering.

With the jittering fixed, it's more obviously when the particle penetrates and then de-penetrates a surface (in the next frame) unofortunately, but that is something we can fix later. This PR is already a huge improvement.

[Calinou]

Tested locally, it works as expected.

However, this PR completely changes the appearance of the trails particles in https://github.com/godotengine/godot-demo-projects/tree/master/3d/particles due to the collision behavior being modified in "fringe" cases:

Before

particle_trail_collision_before.mp4

After (this PR)

particle_trail_collision_after.mp4

I assume this is because this PR changes how particles collide when they spawn within a collision box (even if only slightly). Reducing the particle's collision base size from 0.1 to 0.01 fixes the issue in the particles demo, but I thought I'd highlight it.

[Calinou]

Despite my above comment, I still think this is good to merge. Code looks good to me.

I would highlight this change in the next blog post still, just in case.

[akien-mga]

Thanks!

[Rudolph-B]

@Calinou What version/commit did you use for the before?

[Calinou]

@Calinou What version/commit did you use for the before?

It's master branch from a week ago or so (it's my system-wide godot binary which I regularly rebuild). The particles have looked like this since the initial addition of the particles demo around 4.1.