Now the function utf8_char_size_screen can return -1 when the char is not
printable.
It has a specific behavior for some chars:
- U+0009: value of option weechat.look.tab_width
- U+0001 to U+001F (except U+0009): 1
- U+00AD (soft hyphen): -1
- U+200B (zero width space): -1