That is a good question.

Let's consider the Sphere Mollweid projection for example. Its validity extent is

```
[-18019909.21177587, -9009954.605703328, 18019909.21177587, 9009954.605703328]
```

Question is: how can I get minLon, minLat, maxLon, maxLat from that extent?

A naive approach is to transform the four corners and take the bounding box of the transformed corners (what ol.extent.applyTransform does):

```
> var t = ol.proj.getTransform(ol.proj.get('ESRI:53009'), ol.proj.get('EPSG:4326'));
> var e = [-18019909.21177587, -9009954.605703328, 18019909.21177587, 9009954.605703328];
> ol.extent.applyTransform(e, t);
  [0, -90, 0, 90]
```

That does produce the result one could expect. This is because the transforming `[x,-9009954.605703328]` returns `[0,-90]` and transforming `[x,9009954.605703328]` returns `[0,90]`, whatever the value of `x`. And this actually makes total sense.

So how to get minLon and maxLon from the validity extent?

I can transform `[-18019909.21177587,0]` and `[18019909.21177587,0]` but using `0` is arbitrary and that may not work with any projection. And also:

```
> var t = ol.proj.getTransform(ol.proj.get('ESRI:53009'), ol.proj.get('EPSG:4326'));
> transform([-18019909.21177587, 0])
 [180, 0]
> transform([18019909.21177587, 0])
 [-180, 0]
```

Pay attention to the signs of the results.

So I don't know how to derive minLon, minLat, maxLon, maxLat from our validity extent with a systematic way that will work for any projection.

Please tell me if you have suggestions. It's entirely possible that my understanding of projections is too poor.

(Not that instead of adding `minLon`, `minLat`, `maxLon`, `maxLat` I could add `epsg4326Extent`, which would be more consistent with the existing `extent` property/option.)