[bdf, pcf] Support ISO646.1991-IRV character encoding (aka ASCII). Problem reported by Marek Kašík <mkasik@redhat.com>, cf. https://bugzilla.redhat.com/show_bug.cgi?id=1451795 * src/bdf/bdfdrivr.c (BDF_Face_Init), src/pcf/pcfdrivr.c (PCF_Face_Init): Implement it.