term: insert-mode: handle combining characters correctly

When the client application emits combining characters, for example
multi-codepoint emojis, in insert-mode, we ended up pushing partial
graphemes to the right, for each codepoint, resulting in too many
cells (and with the wrong content) being inserted.

The fix is fairly simple; don't "insert" when appending characters to
an existing grapheme cluster.

This isn't something we can detect easily in print_insert() (it would
require us to do grapheme clustering again). Fortunately, we do have
the required information in action_utf8_print(). So, pass this
information as a boolean to term_print().

Closes #1947

csi.c

@@ -793,7 +793,7 @@ csi_dispatch(struct terminal *term, uint8_t final)
                 const int width = c32width(term->vt.last_printed);
                 if (width > 0) {
                     for (int i = 0; i < count; i++)
-                        term_print(term, term->vt.last_printed, width);
+                        term_print(term, term->vt.last_printed, width, false);
                 }
             }
             break;