* src/buffer.c: add a short test program.
* src/iso2022.c: use hard values instead of GDK defines in the 0 map.  Add
	_vte_iso2022_substitute_single() for performing a single mapping
	operation.
* src/vte.c: use _vte_iso2022_substitute_single() instead of a local mapping
	table when mapping line-drawing charset data, simplifies maintenance
	of the mapping.