(encode_terminal_buf, encode_terminal_bufsize): New

(encode_terminal_buf, encode_terminal_bufsize): New
variables.
(encode_terminal_code): Argument changed.  Encode all
characters at once, and return a pointer to the result of
encoding.
(write_glyphs): Decide coding here.  Adjusted for the above
change.
(insert_glyphs): Likewise.
(term_init): Initialize encode_terminal_bufsize to 0.