Incremental encoding api

[birktj]

With image-rs/image-png#102 and image-rs/image-tiff#23 I would like to:

1. Create a tiff encoder with support for incremental encoding
2. Add incremental encoding to the png encoder

However I am not sure what a good api for incremental encoding would be?

One possibility is to do something like ImageDecoder and simply create a writer that you can write image bytes to. An alternative would be to have some type of Encoder::write_row method.

I am mostly leaning towards a Write interface as it it looks the most like the existing write_image(..., image: &[u8]) methods that most decoders use.

However I figured I should ask if anyone have better ideas or comments on the proposed apis?

[lovasoa]

This would be very useful to me for dezoomify-rs