[optimizer] Improve srcset generation

The current algorithm generates three srcset sources from this list:

```
const SRCSET_WIDTH = [39, 56, 82, 100, .., 2500];
```

Starting at the image width X and picking the next two versions with twice the resolution from the list (> 2 * X and > 3 * X). This does not always make sense. For example:

* intrinsic layout: width defines the maximum width of the image, so we should also provide a smaller version.
* responsive layout: if the given height is small (e.g. for a mobile device screen) it might make sense to generate more than three larger versions.

More investigation needed.

//cc @patrickkettner

No implementation file, test, or entry point is named. Locate the code that selects the three srcset widths, then investigate intrinsic and responsive layout cases against the current SRCSET_WIDTH list. The issue needs a decided algorithm and acceptance criteria before completion can be judged.

Written by the indexing model from the issue text.