Make explicit links between elements that are used to activate the
next element in the graph.
Make subgraphs a special regular node. Make a link from the
subgraph children to the parent so that the subgraph completes when
all the children completed.
Implement a single process function in plugins
Remove many messages in the client node
Move the activation state to the graph
Replace an old scheduler with a new one that works with the new
activation states.
Remove the DISABLED port flags, we explicitly add and remove to
make ports enabled/disabled.
Find first compatible port for link
Remove the node based scheduler and use the spa one.
Don't unlink the ports of an inactive link because then it might be
possible that the scheduler can't recycle a buffer, instead use
a port flag to mark the ports disabled and change to scheduler to
skip those ports.
Add a check against letting the required number of ports become
negative for a node. With certain timing between the main and data
thread, this can otherwise happen.
For nodes where links are unlinked and relinked, a negative number may
mean that the scheduler skips the node and doesn't call
process_output().
Make it possible to configure multiple io areas on a port by giving
an id to set_io.
Add some types to enumerate the supported ids
Make an area to exchange buffers and one to specify pull ranges.
The idea is to make more area types for controlable properties.
Implement enumeration of IO areas in volume.
