Merge: gamnit: customize writing with BMFont

Add format services to `TestSprites` implemented (only) by `BMFont`: `align, valign, wrap, max_width` and `max_height`. You can see the result in the screenshot of the new showcase/test program:

![screenshot from 2017-06-15 15 53 28](https://user-images.githubusercontent.com/208057/27199185-1b9a36dc-51e3-11e7-9700-8dde5724e98a.png)

Pull-Request: #2499
Reviewed-by: Jean Privat <jean@pryen.org>

diff --cc lib/gamnit/bmfont.nit
Simple merge