selection: recognize empty padding cells in a forced linewrap

When printing a multi-column character at the end of the line, and it
doesn't fit, we currently insert a forced line-wrap. This means the
last character(s) on the previous line will be empty, followed by a
multi-column character in the first cell on the next line.

Without special code to handle this, the selection text extraction
code will insert a hard newline, since this is normally the correct
thing to do.

Add a TODO, to consider writing a special place holder value to these
padding cells.

terminal.c

@@ -2399,6 +2399,16 @@ term_print(struct terminal *term, wchar_t wc, int width)
         /* Multi-column character that doesn't fit on current line -
          * force a line wrap */
         term->grid->cursor.lcf = 1;
+
+        /*
+         * TODO: should we insert place holder values in the remaining
+         * cells?  This would allow e.g. text extraction to simply
+         * skip these, instead of trying to recognize a sequence of
+         * empty cells at the end of the line followed by a
+         * multi-column character...
+         *
+         * Might also make text reflow easier, or even more correct.
+         */
     }
 
     print_linewrap(term);