This way we can call process_subtitles without causing the decoded
frame counter to get bumped.
Additionally, this now takes into mention all of the decoded
subtitle frames without fix_sub_duration latency/buffering, or filtering
out decoded reset/end subtitles without any rendered rectangles, which
matches the original intent in 4754345027eb85cfa51aeb88beec68d7b036c11e
.
Signed-off-by: Jan Ekström <jan.ekstrom@24i.com>