combined: Leave the screen_char attributes unchanged.
The previous code displayed the wrong attributes if the combining
characters were at the end of an HTML link. For example:
<a href="#">trickỹ</a> more text <a href="#">second link</a>
(The characters in the first A element are "tricky" and U+0303
COMBINING TILDE.)
Here, when the cursor was not at the first link, ELinks displayed
the y-with-tilde cell as if it were not part of the link.
This happened because ELinks had already changed schar->attr
before set_line saw the space character after the link and
flushed document->combi[].