sixel: purge images at current cursor row

When printing a character, or starting a new sixel image, purge all
images that cover the cursor's current row.

terminal.c

@@ -2128,6 +2128,8 @@ term_print(struct terminal *term, wchar_t wc, int width)
     print_linewrap(term);
     print_insert(term, width);
 
+    sixel_purge_at_cursor(term);
+
     /* *Must* get current cell *after* linewrap+insert */
     struct row *row = term->grid->cur_row;
     struct cell *cell = &row->cells[term->cursor.point.col];