Use Khronos Dataformat descriptors while passing images between processes

[mfrischknecht]

While it still makes sense to use the same image format internally for almost everything, passing a description of the data format with every image could allow us to also provide code to transform images from one format to another (optimally in an entirely separate library). This should increase the flexibility of plugins quite a bit.

Dataformat Spec: https://www.khronos.org/dataformat
"Lightning Talk" during SG15: https://youtu.be/quNsdYfWXfM?t=12m