Fix: update TextTrack to handle SRT file conversion and location mana…

[cyu098723]

Problem Summary

Cause:
After a subtitle was converted/renamed to .srt, the TextTrack object continued to point (self._location) to the original path ending in .mp4. When muxing, mkvmerge received the .mp4 path (which didn't exist) and failed with an "open file error."
Example of the error you showed: mkvmerge tried to open ~/VT-PR/Temp/TextTrack_en-us_subtitle_dialog_0_dec.mp4 but the subtitle was actually in .srt format.

Fixes:
Update self._location when the file is renamed to .srt.
When converting formats (when self.codec != "srt"), I now explicitly write the SRT to a file with the .srt extension, remove the original container file, and update self._location to point to the .srt file.
I kept the behavior of returning the actual path of the final file (now returns .srt when applicable).

How is this the case?
After the fix, TextTrack._location now points to TextTrack_... .srt. In the mux step, Tracks.mux() calls st.locate() and gets the correct .srt path, so mkvmerge won't try to open a non-existent .mp4.

Fix these issues: #144 #162 #182

[chu23465]

You have tested this fix right?