Rasterize a whole string

[Inspirateur]

Hi, I'm making a wordcloud program and I'd like to be able to convert a whole string such as "Hello world" to a bitmap directly, like we can already do with individual chars.
Could this be added to fontdue ?

If this feature is out of scope for the library could you help me in implementing that ?
I'm lost in all the Layout and Metrics parameters and I'm not sure how to interpret them, especially the negative xmin and ymin of Metrics.