Still to do:
 * Cleanly report Xlib errors
 * Setup a scheme for error reporting
 * Compute row height (/ char width ?) on 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
 * Add FontConfig optional support for last-resort fallback font (?)