TTML sub can need video resolution

[Arias800]

Describe the bug
Maybe it's a specific situation, but sometimes muxing can fail due to a pycaption error : "At least one of video width or height must be given as reference"
It happened because, for some STPP file, pycaption need the video resolution to convert the sub correctly.

To Reproduce
Download an STPP file that is affected by this requirement.

Expected behavior
Conversion should work

To confirm the problem, I manually changed subtitle_text = writer().write(caption_set) in subtitle.py to subtitle_text = writer(video_width=1920, video_height=1080).write(caption_set).