Add v4l2sink plugin support

[sirn]

This pull request adds support for obs-v4l2sink plugin which can be installed with the similar manner as NDI/BlackMagic support and solves #65.

I decided to use HEAD of obs-v4l2sink instead of v0.1 as HEAD contains several build fixes and it doesn't seems like there's going to be a new release anytime soon.

I've tested this with several apps, e.g. Google Meet, Flatpak Zoom and it has been working fine.

---

Until this is merged, the steps to use goes among the following line:

1. Install flatpak-builder
2. Clone the repo and switch to appropriate branch v4lsink-support (sic):

   $ git clone --branch=v4lsink-support https://github.com/sirn/com.obsproject.Studio
   $ git clone --branch=com.obsproject.Studio.V4L2Sink https://github.com/sirn/flathub com.obsproject.Studio.V4L2Sink

3. Install a custom OBS Studio (for extension support):

   
   $ cd com.obsproject.Studio
   $ flatpak-builder --force-clean build-dir com.obsproject.Studio.json --user --install
   $ cd ..

4. Install V4L2Sink plugin

   $ cd com.obsproject.Studio.V4L2Sink
   $ flatpak-builder --force-clean build-dir com.obsproject.Studio.V4L2Sink.json --user --install
   $ cd ..

To use the V4L2Sink plugin:

1. Install v4l2loopback DKMS
2. Load the kernel module:

   # modprobe v4l2loopback

3. Launch OBS and go to Tools -> V4l2sink, and select the loopback video input

[flathubbot]

Started test build 19918

[flathubbot]

Build 19918 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/19109/com.obsproject.Studio.flatpakref

[Pshemas]

how do you install this branch v4l2loopback support? I've tried install using instructions in the 1st comment, but when I do: flatpak-builder --force-clean com.obsproject.Studio.json --user --install

I get : MANIFEST must be specified

[sirn]

@Pshemas sorry I miss one argument; fixed now. It should be

$ flatpak-builder --force-clean build-dir com.obsproject.Studio.json --user --install
$ flatpak-builder --force-clean build-dir com.obsproject.Studio.V4L2Sink.json --user --install

[TingPing]

Unlike blackmagic and NDI, this is entirely open source so we can do better!

1. Submit a PR here that only adds the extension point to OBS
2. Create a metainfo file for the extension. Like: https://github.com/flathub/io.github.Hexchat.Plugin.OTR/blob/master/io.github.Hexchat.Plugin.OTR.metainfo.xml
3. Submit it to Flathub: https://github.com/flathub/flathub/wiki/App-Submission

[sirn]

@TingPing That's much better. I'll update this PR to only provides extension point and work on submitting the extension to Flathub. Thank you!

[flathubbot]

Started test build 20799

[sirn]

@TingPing Not sure if I should rename com.obsproject.Studio.V4L2Sink to com.obsproject.Studio.Plugin.V4L2Sink (since NDI/BlackMagicLibs doesn't have one), but I've removed V4L2Sink plugin & only contain the entrypoint here in this PR. Going to submit V4L2Sink to Flathub soon.

[flathubbot]

Build 20799 successful To test this build, install it from the testing repository:

flatpak install --user https://dl.flathub.org/build-repo/19951/com.obsproject.Studio.flatpakref

[flavorjones]

Just wanted to say "thanks" to you both for doing this work!