This adds a basic support to use multiple sound-cards, currently
limited to two devices: In/Out with shared settings.
Advanced setups still have to resort to using the ARDOUR_ALSA_EXT
environment variable
This is intended to prevent crashes when unregister_port() modifies the contents of these
two members at the same time that something else is iterating over them.
This can help when running with very low latency and the
initial process callback is [indirectly] expensive.
E.g. load a heavy session the a RPi4, initial setup can pull
in a lot of data, which blocks the bus.
In particular with the ALSA backend this can lead to poll timeout
which effectively stops the backend.
When recover() successfully re-initializes the device,
processing can continue just like after an x-run.
This can happen during initial session load of "expensive"
sessions (in particular on slow systems, e.g. Raspberry Pi)
usually with synths. Worker thread pulls in many external
files in the background which blocks the bus for a long time.
resulting in a poll-timeout.
Previously this was inherited via PBD.
On MacOS/X, this adds
"-undefined dynamic_lookup -flat_namespace"
and various "-framework .." options to linkflags
Without this flag, .dylibs fail to link usually because
of missing `-lintl` (Undefined symbols: "_libintl_dgettext")
On other systems this is a NO-OP:
CFLAGS_OSX, CXXFLAGS_OSX and LINKFLAGS_OSX
are only set on the darwin platform.
This produces synchronous events on Audio and MIDI ports.
One rvent per second, exactly at every second since engine-start.
MIDI: C-4 Note-on/off (1 sample long)
Audio: +1/-1 transition:
+1 in sync with Note-on,
-1 in sync with Note-off
gcc can recognize various regexps in comments. Since C++17 provides
[[fallthrough]], using /* fallthrough */ consistently seems
appropriate until we switch to C++17.
see also https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html
