Still images not updating when using custom videoFilter
<!-- You must use the issue template below when submitting a bug -->

**Describe The Bug:**
When using a custom videoFilter in the config, still images fail to update. The change that introduced this bug was in release 0.1.18 where the videoFilter value is added to the ffmpeg command rather than the default scale values. Specifically https://github.com/homebridge-plugins/homebridge-camera-ffmpeg/compare/v0.1.17...v0.1.18#diff-aa15bf368787a2fc2fbd3f276bdcca87R188

**To Reproduce:**
Add a custom videoFilter. Mine specifically is "format=nv12|vaapi,hwupload,scale_vaapi=w=1920:h=1080 -bf 0" as I am using hardware encoding for my ffmpeg streaming

**Expected behavior:**
Still images should update in the app occasionally 

**Logs:**
Happy to provide logs however they don't actually report failures. I can however downgrade to 0.1.17 and it starts working again. Anything above 0.1.18 fails to update unless I remove the custom videoFilter

```
Show the Homebridge logs here.
Remove any sensitive information.
```

**Homebridge Config:**

```json
           "platform": "Camera-ffmpeg",
            "videoProcessor": "/custom/built/bin/ffmpeg",
            "cameras": [
                {
                    "name": "Front Porch",
                    "videoConfig": {
                        "source": "-init_hw_device vaapi=intel:/dev/dri/renderD128 -hwaccel vaapi -vaapi_device /dev/dri/renderD128 -rtsp_transport tcp -re -i rtsp://192.168.1.11:80/cam/realmonitor?channel=1&subtype=0",
                        "stillImageSource": "-rtsp_transport tcp -re -i rtsp://192.168.1.11:80/cam/realmonitor?channel=1&subtype=0",
                        "maxStreams": 3,
                        "maxWidth": 1280,
                        "maxHeight": 720,
                        "maxFPS": 30,
                        "maxBitrate": 1024,
                        "vcodec": "h264_vaapi",
                        "videoFilter": "format=nv12|vaapi,hwupload,scale_vaapi=w=1920:h=1080 -bf 0"
                    }
                },
```

**Screenshots:**
<!-- If applicable, add screenshots to help explain your problem. -->

**Environment:**

* **Node.js Version**: v11.6.0
* **NPM Version**: 6.13.0 
* **Homebridge Version**: 1.0.4 
* **Homebridge Camera FFmpeg Version**:  0.1.18 or above fails 
* **Homebridge Config UI X Plugin Version**: 4.19.0 
* **Operating System**: Ubuntu 


<!-- Click the "Preview" tab before you submit to ensure the formatting is correct. -->
