Scalable icons are not used

In my mako config I have:
```
icon-path=/usr/share/icons/elementary
```

When I send a notification with `"newsboat"` as the icon, it doesn't show any icons, although I have the following in my filesystem:
```
/usr/share/icons/hicolor/scalable/apps/newsboat.svg
```
If I supply the full path, it works, of course.

Reading through the code I believe that icons in the `scalable` folder are going to be ignored completely because [this](https://github.com/emersion/mako/blob/master/icon.c#L156) will produce an error and will cause the loop to [move on](https://github.com/emersion/mako/blob/master/icon.c#L165-L167) to the next icon found.

I wonder if it would make sense for icons found in the `scalable` folder to take precedence over others (since they should be able to scale to any desired size) or at least, be accepted as appropriate as soon as the loop finds one.