It seems that BGMDriver was failing to compile on case-sensitive file
systems because BGM_Types.h included "AudioServerPlugin.h" instead of
"AudioServerPlugIn.h". (Lowercase "i".)
I tried building with the project and Xcode on a case-sensitive disk
image and it would fail without this patch. So I figure it should at
least build now. I haven't had time to test Background Music on a system
running on a case-insensitive file system yet, so I added a TODO about
it in TODO.md.
Also, some unrelated tidying up.
In the manual uninstallation instructions, makes the command for
restarting coreaudiod clearer. If the old command had to fallback to
"sudo killall coreaudiod", it wasn't clear to the user that it had
actually succeeded.
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.
