Keep srcset from using hard cropped images

This patch fixes an issue where custom crops of an images with different aspect ratios would be returned as `srcset` candidates.

For example, a theme or plugin might add a custom size as follows:

```
add_image_size( 'custom-square', 500, 500, true );
```

In that case, we wouldn't want those images used in our `srcset` attribute, only soft-cropped images (i.e., images with the same aspect ratio).