Allow overriding font ascender and descender.

Provides an easy option for influencing vertical metrics, which may be
useful for implementing replaced and inline-block elements.

Prepare for vertical-align in BoxOptions.

Propagate BiDi levels to layout algorithm.

BREAKING: Text direction of the root box is now correctly used for
ordering fragments. Previously, the order was always LTR.

Add required textDirection to TextOptions.

Define BoxOptions and TextOptions for formatting.

Differentiating the two record types reflects the fact that the root
inline box cannot be styled but its text contents can.