Add Batcher utility (djeedai#211)

Add a new internal `Batcher` utility used to batch particle effects for
submission to GPU. This replaces the ad hoc code previously used
directly in the `prepare_effects()` system, which was hard to maintain
and to test, and the source of numerous bugs and regressions.

src/attributes.rs

@@ -1068,7 +1068,7 @@ impl ParticleLayout {
     /// The layout is immutable. This is mostly used as a placeholder while a
     /// valid layout is not available yet. To create a new non-finalized layout
     /// which can be mutated, use [`ParticleLayout::new()`] instead.
-    pub fn empty() -> ParticleLayout {
+    pub const fn empty() -> ParticleLayout {
         Self { layout: vec![] }
     }