Optional bulk form of update, applied to particles[0..count).
The per-particle update is reached through one call site shared by every
behavior type, which leaves it megamorphic: V8 can neither inline it nor
specialise the generator calls inside. A behavior that implements this
instead gets one call per frame and a loop it owns, where those calls are
monomorphic. Implementations must skip dead particles exactly as the
per-particle path does, and must produce identical results.
Behaviors that do not implement it keep working unchanged.
Staticfrom
Apply size to particles based on their life.