codecalpha: Fix return flow propagation

This merge request address issue with the alpha decoder whenever a downstream EOS is sent. This MR fixes flow propagation and move away from multiqueue, though this last one is to workaround some decode error produce by vp9dec after the flow EOS have gone through. I could not figure-out why this would cause the following frames to be corrupted, but what I notice is that multiqueue was being more flexible then I wanted, and would lead more then 10 buffers being queued, which is the threshold for decoders to produce error on the bus.

Fixes #1590 (moved)