Hi,
I was building a texture packer with sharp, and i use sharp.overlayWith to pack small pictures to a big picture.
I found the way to overlay multiple pictures to a picture in #405. But it seems this will increase memory usage to a huge number and memory will not be release.

Before pack textures: memory usage 88.8 m

After pack textures: Memory usage 1.67 g

I have also disable the cache with sharp.cache(false), but it has no effect.

The example is here: https://github.com/2youyou2/electron-pitfalls
Please follow these steps:

• git clone https://github.com/2youyou2/electron-pitfalls.git
• npm install
• npm start sharp-memory

Thanks for looking this.