Customizable particle attribute layout

[djeedai]

The current particle attribute layout is hard-coded into the GpuParticle type:

bevy_hanabi/src/render/mod.rs

Lines 1117 to 1129 in 0f7494d

	/// GPU representation of a single particle stored in a GPU buffer.
	#[repr(C)]
	#[derive(Debug, Copy, Clone, Pod, Zeroable, ShaderType)]
	struct GpuParticle {
	/// Particle position in effect space (local or world).
	pub position: [f32; 3],
	/// Current particle age in \[0:`lifetime`\].
	pub age: f32,
	/// Particle velocity in effect space (local or world).
	pub velocity: [f32; 3],
	/// Total particle lifetime.
	pub lifetime: f32,
	}

It contains the position and velocity of the particle, its age, and its maximum lifetime after which the particle dies.

Although this is the most common set of attributes, some effects require more per-particle attributes, like the particle size (per-particle size variation/randomness), or its color, among many examples. To unlock building such effects, the particle layout should instead be dynamically determined by the set of modifiers which define the effect. This allows enabling a wide range of new effects while keeping the per-particle data as small as possible.

The idea is to define an Attribute type representing a single attribute of a particle, with a name (e.g. "position") and a value (e.g. vec3<f32>), and compose the minimal set of attributes needed per effect into a particle layout which determines how the per-particle data is encoded in the GPU buffer of particles.