djeedai / bevy_hanabi / 8396839848

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Implement simple ribbon rendering. (#298)

This adds a new render modifier, the `RibbonModifier`. When it's
present, quads are drawn between particles that are part of a trail
created with the `CloneModifier`, instead of directly at the particle
locations.

Internally, particles are threaded into a doubly linked list, with the
`PREV` and `NEXT` attributes. I chose a linked list over fixed-size
trails like the Unity VFX graph uses because (a) during ribbon
rendering, a particle only needs to know the positions of its immediate
neighbors; (b) linked lists better support variable-length trails. We
need a doubly linked list instead of a singly linked list because (a)
more sophisticated ribbon rendering will want to know about both the
next and previous particles in order to implement line joins properly;
(b) a singly linked list has difficulty dealing with dangling pointers
to dead particles.

Each ribbon quad is initially oriented toward the average normal
(`nlerp`) of the two particles that define it, modulo Gram-Schmidt
normalization. Eventually, ribbon quads should be extended to support
meshes with line joins, as well as perhaps curves, but the current
approach should do for an initial implementation.

A new example, `ribbon`, has been added.

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