`rr.Image` constructor and `columns()` behave differently

[grtlr]

When constructing an rr.Image via the constructor, the ImageFormat is set derived automatically from the data that you put in (e.g. a NumPy array). The same is not tru for the rr.Image.columns() method, which is used together with send_columns.

It would be nice to unify the behavior between those two APIs. At the very least, we should point that out in our docs.