Update GC cleanup with exclusion list

garbage-collector.ts

/**
 * Delete any objects tagged for garbage collection.
 */
export const cleanup = (exclude?: Array<Manifold | CrossSection>) => {
  const deleteUnlessExcluded = getDeleter(exclude);
  for (const obj of memoryRegistry) {
    // decompose result is an array of manifolds
    if (obj instanceof Array) 
      for (const elem of obj) deleteUnlessExcluded(elem);
    else 
      deleteUnlessExcluded(obj);
  }
  memoryRegistry.length = 0;
};

/**
 * Return an object deleter that honors an optional exclusion list.
 */
const getDeleter = (exclude?: Array<Manifold | CrossSection>) => {
  if (exclude) {
    const excludeSet = new Set<Manifold | CrossSection>(exclude);
    return (obj: Manifold | CrossSection) => {
      if (!excludeSet.has(obj)) obj.delete();
    };
  }
  return (obj: Manifold | CrossSection) => obj.delete();
};