Face method 'bounding-box' should return a four-element array or list as in Font::AFM

[tbrowder]

Similar to Font::AFM's FontBBox.

[dwarring]

I'm relucent to change bounding-box given that it's already in use.

Instead, I'm adding a FontBBox method that returns an array. Also adding documentation that bounding-box returns a struct with x-min, y-min, x-max and y-max accessors.

Hope this suits.

[tbrowder]

That's fine, David. Thanks!