Combine loadImage and addImage

Nobody has claimed this yet.

It's not the end of the world, but I find this pattern clunky and hard to remember:

```
map.loadImage('assets/images/marker.png', (error, image) => {
map.addImage('marker', image);
});
```

It could just as easily be:

```
map.addImage('marker', 'assets/images/marker.png');
```

I would have thought that the primary use case for `addImage` is fetching an image specifically for the purpose of adding it to the map, rather than using some existing image that happens to exist in the DOM.

Start by tracing the existing loadImage and addImage entry points and their public API behavior. Compare how each handles image URLs, loading errors, and already available images; the work is done when the proposed combined call has clear behavior and corresponding coverage or documentation.

Written by the indexing model from the issue text.