Add NVIDIA support for FFmpeg in Docker and enhance chat rendering functionality

- Introduced a new docker-compose.nvidia.yml for NVIDIA GPU support.
- Updated dockerstart.bat to allow optional NVIDIA runtime.
- Enhanced ContentDownloader to manage chat rendering status and font settings.
- Improved hardware acceleration detection in utils.py.
- Added tests for hardware acceleration and chat rendering behavior.

Co-authored-by: Copilot <copilot@github.com>

README.md

@@ -86,6 +86,17 @@ Or use the Windows helper:
 
 That batch launcher mirrors the old pattern and expands to a compose `run` command, so you can test any streamer manually.
 
+If the host has the NVIDIA Container Toolkit installed and you want FFmpeg/NVENC inside the container, use the optional NVIDIA override:
+
+```powershell
+.\dockerrebuild.bat
+.\dockerstart.bat --nvidia vinesauce --verbose
+```
+
+The image built by `.\dockerrebuild.bat` already includes the NVIDIA-capable FFmpeg/container toolchain. The optional [docker-compose.nvidia.yml](docker-compose.nvidia.yml) layer is only for runtime GPU passthrough: it requests `gpus: all` and sets `NVIDIA_VISIBLE_DEVICES` plus `NVIDIA_DRIVER_CAPABILITIES=compute,utility,video` for the container.
+
+On systems without NVIDIA support, keep using the normal command without `--nvidia`; the image still builds the same way, it just runs without GPU passthrough.
+
 ### Healthcheck and smoke tests
 
 - Container healthcheck command: `python twitch-archive.py --healthcheck -u vinesauce`