video: Add new text layout API, baseclass, and elements
- Adding GstTextLayout and GstVideoSubtitleMeta for subtitle information (e.g., string, text layout, attributes) signalling
- Add subtitlemux element to aggregate subtitle and attach GstVideoSubtitleMeta to output video buffers
- New baseclasses for textlayout rendering
- Update dwrite plugin to use new baseclass/API
- Add pango2 plugin based on new baseclass/API
Edited by Seungha Yang