Various improvements for SDH filter

[forthrin]

Important Information

System Version: macOS 14.2.1 (23C71)
Model Identifier: MacBookAir10,1
mpv version: git-2024-01-06-d8c2e33
FFmpeg version: git-2023-12-11-5fdbc2a

Reproduction steps

mpv --sub-filter-sdh=yes --sub-file=sample.srt any.mp4

Expected behavior

1. "I'll sing something for you."
2. Blank
3. "Stay a while..." if sub-filter-sdh-harder=yes
4. "Stay forever!"
5. As-is
6. As-is
7. As-is
8. "Lowercase -all- is a common speaker"
9. "Lowercase -both- is a common speaker"

Actual behavior

1. As-is
2. As-is
3. As-is
4. As-is
5. Shows only line two
6. Shows only line two
7. Shows only line two
8. As-is
9. As-is

Sample files

1
00:00:00,000 --> 00:00:01,000
I'll sing something for you.
-♪ Yesterday...

2
00:00:01,000 --> 00:00:02,000
♪ Yesterday...
All my troubles...

3
00:00:02,000 --> 00:00:03,000
Dr. No & Man #1:
Stay a while...

4
00:00:03,000 --> 00:00:04,000
DR. NO / MAN #2:
Stay forever!

5
00:00:04,000 --> 00:00:05,000
‐ x2010
‒ x2012

6
00:00:05,000 --> 00:00:06,000
– x2013
— x2014

7
00:00:06,000 --> 00:00:07,000
― x2015
− x2212

8
00:00:07,000 --> 00:00:08,000
all: Lowercase -all- is a common speaker

9
00:00:08,000 --> 00:00:09,000
both: Lowercase -both- is a common speaker