Handle unbounded processing time for generating content

[laurent-radoux]

Currently, there is very hard constraint on the processing time which cannot exceed the time between two frames.

This is due to the nature of the synchronisation between the RX and TX threads where the processing is never parallelized.
It implies that should the processing be longer, frames will be constantly dropped.

The synchronisation needs to be adapted in order to make it possible for multiple RX frames to be processed at the same time while still ensuring that the latency between the live content and the generated overlay is minimal.