term: append zero-width grapheme breaking characters to previous cell

When compiled with grapheme clustering support, zero-width characters
that also are grapheme breaks, were ignored (not stored in the
grid).

When utf8proc says the character is a grapheme break, we try to print
the character to the current cell. But this is only done when width >
0. As a result, zero width grapheme breaks were simply discarded.

This only happens when grapheme clustering is enabled; when disabled,
all zero width characters are appended.

Fix this by also requiring the width to be non-zero when if we should
append the character or not.

Closes #1960

terminal.c

@@ -4153,7 +4153,7 @@ term_process_and_print_non_ascii(struct terminal *term, char32_t wc)
         if (grapheme_clustering) {
             /* Check if we're on a grapheme cluster break */
             if (utf8proc_grapheme_break_stateful(
-                    last, wc, &term->vt.grapheme_state))
+                    last, wc, &term->vt.grapheme_state) && width > 0)
             {
                 term_reset_grapheme_state(term);
                 goto out;