Minor video input device issues

```
<`michiru> does someone have a device with 2 cams?
<`michiru> noticed uTox displays video from the second one even though the first one is selected by default
<`michiru> but it works fine after changing it manually
<`michiru> also video preview sometimes doesn't show
<`michiru> win8.1
```

(`video preview` in that context = the video preview window)
