decoder: fix Closed Caption decoding

The change in 61750113 was not equivalent to the tests that were done before.

After this patch the background still doesn't look correct, but the text is intelligible.

The background is correct in the 2023/12/04 windows nightly build. For some reason it's not on my locally compiled master version...

I was checking the wrong date. The background went away between the 2023/11/10 and 2023/11/11 nightlies.

fe415420f3ace0b91636884626ff1ddd6ef8c360 is the first bad commit
commit fe415420f3ace0b91636884626ff1ddd6ef8c360
Author: Steve Lhomme <robux4@ycbcr.xyz>
Date:   Tue Oct 24 11:13:47 2023 +0200

vlc_filter: return a non-text subpicture region when rendering text regions

We no longer need to ugly hack that marks the output region as marked
and we don't mix the input and output regions.

include/vlc_filter.h                        |  7 ++-
modules/text_renderer/freetype/freetype.c   | 77 +++++++++++++----------------
modules/text_renderer/nsspeechsynthesizer.m | 10 ++--
modules/text_renderer/sapi.cpp              |  8 ++-
modules/text_renderer/svg.c                 | 39 ++++++++-------
modules/text_renderer/tdummy.c              |  6 +--
src/video_output/vout_subpictures.c         | 32 +++++++-----
test/src/input/decoder/input_decoder.c      |  6 +--
8 files changed, 94 insertions(+), 91 deletions(-)