Add `sub-rotate` Option for Subtitles

Expected behavior of the wanted feature

Description:
Currently, when rotating the video with "shift+r video-rotate 90 180 270 0" (input.conf), subtitles do not rotate accordingly, causing misalignment. A solution like the following would allow subtitles to rotate with the video:
shift+s sub-rotate 90 180 270 0
This would prevent the issue where subtitles stay static while the video rotates.

Current Workaround:
Using the vf=sub filter rotates subtitles by burning them into the video. However, it only works with certain hardware encoders and has issues like subtitle stretching. Additionally, vf=sub is expected to be deprecated in the future, making this solution unreliable.

Example Issue:

• Star Wars: The Clone Wars for example has
• Original Resolution: 1280x720 (16:9)
• Output Resolution: 1694x720 (2.35:1)
• Subtitles appear stretched (see screenshots):
  • Normal:
  • Stretched [vf=sub]:
  • Stretched and Rotated [vf=sub]:

Environment:

• mpv version: v0.39.0
• OS: Windows 10
• My Config: mpv-config

Related Issues:

• #6400 (Dec 2018)
• #14372 (Jun 2024)

Alternative behavior of the wanted feature

No response

Log File

No response

Sample Files

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Start by reviewing the input.conf bindings shown in the issue and the related discussions in issues #6400 and #14372. Define how a sub-rotate option should interact with video rotation and subtitle rendering, then verify that subtitles rotate without the stretching seen with vf=sub.