Support for WebVTT format

[ferjm]

Spec: https://w3c.github.io/webvtt

[ferjm]

It seems that GStreamer has a parser for this format https://gstreamer.freedesktop.org/data/doc/gstreamer/head/gst-plugins-base-plugins/html/gst-plugins-base-plugins-subparse.html

[iulianR]

I talked with @jdm on Matrix and he said it's OK if I try to implement VTTRegion WebIDL interface. I managed to do that (some more tests pass, but I probably missed some things) and I pushed my changes here. Adding VTTCue seems straightforward as well and I would like to do that as well, and afterwards see what else I can do.

Before starting on VTTCue I thought I would mention here in case someone else might think about starting work on this.

[jdm]

Some brief investigation into how webkit + gstreamer integrates VTT:

• https://webkit-search.igalia.com/webkit/rev/57ed6cedb51e4f83d724ad2e147927cd6376e906/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp#2990-3013
• https://webkit-search.igalia.com/webkit/rev/57ed6cedb51e4f83d724ad2e147927cd6376e906/Source/WebCore/platform/graphics/gstreamer/MediaPlayerPrivateGStreamer.cpp#1372-1461
• https://webkit-search.igalia.com/webkit/rev/57ed6cedb51e4f83d724ad2e147927cd6376e906/Source/WebCore/platform/graphics/gstreamer/InbandTextTrackPrivateGStreamer.cpp#98-124
• https://webkit-search.igalia.com/webkit/source/Source/WebCore/html/track/WebVTTParser.cpp
• it seems like the first step is setting the text-sink property on the pipeline, and then setting the up the sink object to handle the new-sample signal as well as accept appropriate caps for the VTT data
• after that, whenever a text sample is obtained, it needs to be parsed by a custom webvtt parser
• I wonder if we could reuse https://github.com/expenses/vtt for the parsing

[jdm]

To be clear, the gstreamer-related work will need to happen in https://github.com/servo/media:

• the actual gstreamer bits will be part of https://github.com/servo/media/blob/master/backends/gstreamer/player.rs (presumably)
• then we'll need a backend-independent interface in https://github.com/servo/media/blob/master/player/lib.rs that hooks into the gstreamer backend

Once the interface exists, it can be used to interact with the DOM side of Servo via this field.