Wrap declare_imageio_format

[scott-wilson]

The API is slightly challenging to work with due to passing callbacks from Rust to C++ slightly challenging. Will require some thinking/change how the API works from the Rust side.

[lgritz]

Just leave it out. The purpose of that call is to dynamically add C++ implementations of a format readers/writers. I think we'd want a different mechanism if and when we allow Rust implementations of formats. Without that, I can't see how this would be of use to Rust clients of the library.

[scott-wilson]

Sounds good. I was thinking of revisiting this once we start implementing creating Rust powered formats. Maybe do that all via a C interface? Don't know, will play around with that.