local: Shut down the media renderer pipeline if there is no source
owr: Refactor common code from both media sources to the base class

It now creates the inter*src and converters as required.
local: Drop leaky queues in the renderers and set the sink to async=TRUE

The inter*src already has the leaky behaviour we want and we can safely
do async state changes now as our renderers are self-contained pipelines.
transport: Start the transport agent pipeline when the first media session is added

No point in having the transport agent in PLAYING before that.
transport: Make the input tee non-leaky

This is not required anymore as the inter*src are leaky already.
local: Remove the converters from the image renderer

This is now all handled inside the sources at a central place.