freetype: use UCS-4 encoding on OS/2, too

This fixes subtitle texts are not shown correctly on OS/2.

Freetype module expects UCS-4 string. However, OS/2 iconv() does not
support UCS-4 encoding, so UCS-2 encoding is used on OS/2. Because of
this mis-match, subtitle texts on OS/2 are mis-interpreted. As a result,
subtitle texts are not shown correctly.

(cherry picked from commit 89b561ea)

  Conflicts:
    modules/text_renderer/freetype/freetype.c
    modules/text_renderer/freetype/text_layout.c