Fix horizontal gaps in montage mode

[qustrolabe]

There are black horizontal 5px wide stripes between rows in montage mode. This happens because there are additional 5px added to height for index mode text.

text_area_h = fh + 5;
tot_thumb_h = opt.thumb_h + text_area_h;

These gaps definitely shouldn't appear in montage mode as no text being printed there, and it makes usage of --limit-height flag hard because of 5px added every row.

Fixed by adding condition to add text_area_h to tot_thumb_h only in index mode.

Example with 4 separate images --thumb-width 256 --thumb-height 256 --limit-width 512
Before:

After: