better handling of "Frame source returned empty result"

I wanted to do a quick test to make sure alpha-channel video is composited correctly but I'm encountering some strange behavior. It might just be that the test file I am using is corrupt but it plays fine in mpv.

https://raw.githubusercontent.com/kinetiknz/nestegg/master/test/media/dancer1.webm

I think it is an ffmpeg issue but it's very minor. However, I think there should be better handling of this specific situation:

with

      layers: [{ type: "video", path: "./dancer1.webm" }],

~/amp [2] 🁽 node test.js ; and mpv customFabric.mp4
320x180 15fps
createFrameSource video clip 0 layer 0
createFrameSource fabric clip 1 layer 0
Frame source returned empty result
Frame source returned empty result
Frame source returned empty result
Frame source returned empty result
Frame source returned empty result
(no end to Frame source returned empty result)

I'm not sure what the best option is but it seems like it should exit after some time rather than going on forever

Reproduce the issue with the test.js invocation and the dancer1.webm file, then trace the frame-source handling that emits “Frame source returned empty result.” Done means the empty-result condition no longer runs forever and the command exits or otherwise handles the failure after a bounded period.