BGMApp is failing to build for some people with the Clang error message
"Could not read profile: Unsupported profiling format version". See #4.
I think this might be caused by building with an older version of Xcode,
but I'm far from sure about that. BGMApp uses <0.5% CPU, so it's not
worth using an optimization profile if it's causing anyone problems.
The "install" action for BGMXPCHelper now checks the owner/permissions
of the installation directory. It also installs BGMXPCHelper's
launchd.plist and "bootstraps" it.
The BGMApp project now builds an XPC service bundle called BGMXPCHelper,
which vends a Mach service that BGMApp and BGMDriver can use to
communicate. This will hopefully be useful for some of the tasks HAL
notifications aren't suited to.
In this commit, BGMDriver uses the XPC helper when starting IO, to wait
until BGMApp is ready for playthrough. BGMApp can only start playthrough
when the output hardware is ready for IO. BGMDriver can now tell the HAL
when we're ready for IO, which means we don't have to keep the output
hardware running all the time (or drop frames or increase latency).
The end result is that playthrough doesn't waste CPU time while idle any
more. This also means that now playthrough won't prevent the system from
sleeping when idle.
