There are two known cases where read-only or non-existing sink input volume is
relevant: passthrough streams and the planned volume sharing logic.
Passthrough streams don't have volume at all, and the volume sharing logic
requires read-only sink input volume. This commit is primarily working towards
the volume sharing feature, but support for non-existing sink input volume is
also added, because it is so closely related to read-only volume.
Some unrelated refactoring in iface-stream.c creeped into this commit too (new
function: stream_to_string()).
This adds a PA_VOLUME_IS_VALID() macro for checking if a given
pa_volume_t is valid. This makes changes to the volume ranges simpler
(just change PA_VOLUME_MAX, for example, without needing to modify any
other code).
* Make the dbus object constructors take a pa_dbusiface_core pointer
as an argument. Remove the path_prefix argument.
* Expose the core object path as a constant in protocol-dbus.h.
* Move the core interface name constant from iface-core.h to
protocol-dbus.h.
