Fix WebVTT cue setting preservation

[pdjdev]

Problem

WebVTT cue settings could be silently lost or changed after loading and saving a file in Subtitle Edit.

In particular, commonly used settings such as align:left were not preserved. Cue positions that did not exactly correspond to Subtitle Edit's configured ASS alignment presets were inferred from broad coordinate ranges and converted to a nearby {\an1}–{\an9} tag.

For example, saving this file again via Save As... without any changes:

WEBVTT

00:00:20.943 --> 00:00:21.730
♪ If this is it ♪

00:00:21.731 --> 00:00:23.356 position:3% line:5% align:left
🎵 If This Is It
By Huey Lewis And The News

00:00:23.357 --> 00:00:25.900
♪ Please let me know ♪

could produce:

WEBVTT

00:00:20.943 --> 00:00:21.730
♪ If this is it ♪

00:00:21.731 --> 00:00:23.356 position:20% line:20%
🎵 If This Is It
By Huey Lewis And The News

00:00:23.357 --> 00:00:25.900
♪ Please let me know ♪

This both removes align:left and changes the author's intended position.

Cause & Fix

The previous GetPositionInfo() logic converted arbitrary position and line values to ASS alignment tags based on coordinate ranges. This is a lossy conversion because WebVTT cue positioning is more expressive than the nine discrete ASS alignment positions.

This PR replaces that inference with GetAssAlignmentTagFromCueSettings().

• Cue settings are converted to {\an1}–{\an9} only when the extracted cue settings exactly match one of the configured SubtitleSettings.WebVttCueAn1–WebVttCueAn9 values.
• Non-matching cue settings are preserved instead of being approximated to an ASS alignment tag.
• When an ASS alignment tag is added in Subtitle Edit, its configured WebVttCueAn* value overrides existing cue settings during WebVTT export.
• Cue settings that were converted to an ASS alignment tag are no longer retained as stale metadata. Removing the tag therefore also removes the corresponding cue settings.
• Applied the same behavior to regular WebVTT, numbered WebVTT, and MP4 vttc parsing.
• Removed the range-based WebVTT.GetPositionInfo() method.

For example, when an editor adds {\an7} to a cue:

{\an7}🎵 If This Is It
By Huey Lewis And The News

and SubtitleSettings.WebVttCueAn7 is configured as:

position:20% line:20%

the exported WebVTT cue uses that configured placement:

00:00:21.731 --> 00:00:23.356 position:20% line:20%
🎵 If This Is It
By Huey Lewis And The News

Test

Added regression coverage for:

• preserving non-preset cue settings, including align:left;
• converting exact preset matches to ASS alignment tags;
• removing a loaded ASS alignment tag without retaining stale cue settings;
• ASS alignment tags overriding existing cue settings on save;
• preserving non-preset cue settings in numbered WebVTT files.

Tested on Windows 11 x64 with a local Release build.

WebVttTest: 14/14 passed.

Note

WebVTT and ASS/SSA have different styling and positioning models, so mapping between them has inherent limitations. A dedicated styling system and editor UI for WebVTT could be a useful future improvement.