Still to do:
 * Cleanly report Xlib errors
 * Setup a scheme for error reporting
 * Cleanup bounding box handling: only return length, and compute height on a
   font-wide basis?
 * In dt_load, gracefully handle case when one face refuses to load (ie. free
   already allocated stuff before returning an error)
 * Support for faces other than the first
 * Document
 * Fix Fantasque Sans Mono ascent/descent (or is it a font issue?)