(Qja, Qko, Qzh): New symbols.

(Qja, Qko, Qzh): New symbols.
(syms_of_w32font): Initialise them.
(font_matches_spec): Use them to filter by language.
(recompute_cached_metrics): Remove function.
(compute_metrics, clear_cached_metrics): New functions.
(w32font_encode_char): Use them to manage metric cache.
(w32font_text_extents): Cache metrics for all glyphs on demand.
Delay converting glyph indices to WORD until needed.
(w32font_open_internal): Initialize metric cache to empty.
(registry_to_w32_charset): Charset should always be a symbol.
(fill_in_logfont, list_all_matching_fonts): Family should always be a symbol.