term: print: reset combining characters for this cell

This is the *only* place combining characters are reset. In
particular, we do *not* reset them in a normal cell erase.

This is a performance design decision - clearing the combining
characters in erase is way too slow.

This way, we clear it only when we *have* to. Anything looking at the
combining characters must first ensure the base character is not 0.

terminal.c

@@ -2295,6 +2295,7 @@ term_print(struct terminal *term, wchar_t wc, int width)
     cell->wc = term->vt.last_printed = wc;
     cell->attrs = term->vt.attrs;
 
+    row->comb_chars[term->grid->cursor.point.col].count = 0;
     row->dirty = true;
     cell->attrs.clean = 0;