Image shape validation

Unlike some other packages in the `ndarray-*` ecosystem, `ndarray-pixels` requires images passed to `savePixels` to have certain supported shapes. I'm only committed to supporting 4-channel images, but support for some other shapes might be handled reasonably well by the logic in `common.ts` —

- https://github.com/donmccurdy/ndarray-pixels/blob/main/src/common.ts

That code is based on [`save-pixels`](https://github.com/scijs/save-pixels), which this is a fork of, so I'd need to look more closely to see whether anything other than 4-channel images should really be supported.

Whatever the case, I'd like to add documentation setting expectations for the input shape, and throw clear errors when those expectations are not met.