Add a new builtin module that allows the creation of a custom MediaStream

[mikepavone]

This is the electron/chromium half of the new video pipeline. The integration is a little funny due to the way electron handles customizing chromium. Essentially there are patches to the chromium tree which are very flexible, but difficult to maintain and there are builtin node modules. The latter is easier to maintain, but our need to output real MediaStream objects means we can't use the normal context bridge which means we need to handle installing the function into the main context ourselves.

A couple other notes on this:

• Currently only supports i420 and shared textures (kNative, windows only), but I think that's all we need for our purposes currently
• Shared texture support needs some work before it works properly, but since those are only useful for screen capture preview, it doesn't block getting an initial version out
• Exposed interface is purely C, in part because we used to use different compilers for electron and discord_voice. Might not be necessary now that everything is clang

[eiz]

generally seems 👍 . minor comments

[eiz]

I know we're not using the shared texture path atm but it's surprising to me that we'd do this bit per-frame. Is it cheap?

[mikepavone]

Not sure about the cost. The shared texture path was very much in the "try to get it to work at all" phase when I paused work on it. Definitely would be better to avoid this setup for textures we've already seen.

[adill]

tbh i'd prefer if this was a chromium patch and exposed by default (it certainly feels "off" to be reaching in to and extending blink like this from electron), however, in the end i don't think it matters much either way. (i.e. so we'd just new DiscordVideoStream(streamId))

keeping a strictly C interface is a good call either way, i'd say 👍