customProgramCacheKey() {

		return this.onBeforeCompile.toString();

	}
In case {@link Material#onBeforeCompile} is used, this callback can be used to identify
values of settings used in `onBeforeCompile()`, so three.js can reuse a cached
shader or recompile the shader for this material as needed.

This method can only be used when rendering with {@link WebGLRenderer}.

@return {string} The custom program cache key.