Keep track of queued data in the stream
Pass delay field around to make it possible to know about the raw
software read/write pointer and the hardware one.
Start stream position at 0
Reorganize the io parameter ids and objects. Make separate enumerations
for buffer, control, input and output properties.
Add a volume output property to export-source. This is still unused
but will eventually be routed to a PropsIn io area where it can
control the volume of a mixer, for example.
Add control properties to audiotestsrc for frequency and volume
and add a test program.
Add size parameter to set_io for additional checks and to make it
easy to know how much control data we can write.
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.
