Visual and physics improvements for pappus

[Tientuine]

The pappus prefab could use a bit of fine tuning.

• [x] Narrower stem would look better.
• [x] More consistent and performant particle filaments.
• [x] Gravity so a released pappus will drift to the ground when the wind is absent/calm.
• [x] Related to the above, collider must be adjusted so center of mass is at the base.
• [x] Fix bug where material does not render correctly on particles after replay/restart during running scene.

Update: Bug is not a bug in our implementation, it's actually a Unity bug that only affects the Game View. I verified that if we make a Build it is not affected by the bug. Reference: https://stackoverflow.com/questions/69592234/unity-after-loadscene-and-lighting-and-rendering-not-working-properly