/**
 * Generate small canvas textures (scanlines, vignette, glow, …) and
 * register them with the scene's TextureManager.
 *
 * Cached by key: a second call with the same key reuses the existing
 * texture, so multiple overlays/scenes share one.
 *
 *   canvasTexture(scene, '__glow_cyan', 256, 256, (ctx, w, h) => { ... });
 */
export function canvasTexture(scene, key, w, h, draw) {
  if (scene.textures.exists(key)) return key;
  const canvas = document.createElement('canvas');
  canvas.width = Math.max(1, Math.round(w));
  canvas.height = Math.max(1, Math.round(h));
  const ctx = canvas.getContext('2d');
  draw(ctx, canvas.width, canvas.height);
  scene.textures.addCanvas(key, canvas);
  return key;
}