Video encoding pauses when a game goes under 30ish fps

## Description
When a game goes under 30ish fps the video encoding pauses until the game goes back over 30fps. Audio still gets send to the headset, and the headset tracking data (headset and controllers) gets send to the host machine. This happens with both hardware and software encoding. The pause of new video frames causes the client to reproject the old frames.

I made a short [video](https://youtu.be/nuHcbiOgGeI) showing what happens. I simulated the game going under 30 fps using amd's chill mode to limit the frame rate, but the outcome it still the same.
Notes on the video:
- Task Manager shows the encoding usage, this would normally go to 0 but because of the screen recording it just goes down a little bit.
- Top game window is Half Life Alyx on my pc, as you can see the rendering and tracking continues. Although stutter because of low fps, which is expected.
- Bottom game window is the view from the headset captured through scrcpy.
- Audio continues to stream to the headset, only video doesn't.

I tested all the way back to version `v20.8.1`, and the most night version `v21.0.0-dev07+nightly.2025.07.10`. All versions between `v20.8.1` and `v20.14.0` have the issue, and probably earlier versions too but didn't test that far back. The tested Nightly version also had the same issue.

This wouldn't be a massive issue in most games, but in VRChat it can often happen. And then you would have to go to your pc to close VRChat, or use the SteamVR dashboard to try and navigate the in-game menus. But it is annoying that it happens with ALVR, other streaming solutions like SteamLink don't have this issue.

## General Troubleshooting
- [x] I carefully followed the instructions in the [README](https://github.com/alvr-org/ALVR/blob/master/README.md) and successfully completed the setup wizard
- [x] I read the [ALVR GitHub Wiki](https://github.com/alvr-org/ALVR/wiki)

## Environment

### Hardware

**CPU**: Intel i7 14700k

**GPU**: AMD RX 7900XT

**GPU Driver Version**: 25.6.1

### Installation
**ALVR Version**: v20.14.0

**ALVR Settings File**: https://pastebin.com/WudYWBGb

**SteamVR Version**: v2.12.5

**Install Type**:
- [x] Packaged (`exe`, `deb`, `rpm`, etc)
- [ ] Portable (`zip`)
- [ ] Source

**OS Name and Version**: Windows 11 23H2 (OS Build 22631.5189)