encoder: flicking between multiple sources in recording

To reproduce:

1. Add a screencast source
2. Add a webcam source
3. Click record

Expected: 

The webcam source is always visible above the screencast source as it was added after it

Actual:

There is some flickering where the screencast source is visible over the webcam source. 

NOTE: This behavior isn't present in the preview image, only in the recording.
