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:

[tylerdq]

Maintainers, could you merge this PR? feh is the only program that is working well for me to make montages of large image sets and the black bars kind of kill it. So close to a perfect implementation.

[tylerdq]

I can confirm that building from source with these commits applied produces the intended behavior.