Add Native images to Diagrams.TwoD.Image

Add a type for "Native" images that can be hooked by other packages.

This makes it possible to write libraries like:
 https://taruti.github.io/diagrams-cairo-raster/

The issue with using "Embedded" images is that they are typically not in the pixel format that the backend likes. When using images as tiles (the same image is rendered many times) doing a costly conversion step on each "render" is not feasible.

This minimal support for "Native" images makes it possible to write high-performance native backend specific raster image functionality.
This comment should be updated as well.