freetype: avoid very large fonts in portrait mode

Either when the video is in portrait mode or when the subtitles are shown in the black bars and the video area is in portrait mode, the text can become very large just because we apply a percentage base on the output height.

We switch mode when the height becomes bigger than the width so there's a continuity in the ratio when growing/shrinking the video in one direction or the other.

Before:

After: