Make buffer timestamps more accurate and, more importantly, actually
representative of the MIDI events timing.
Previously, buffers were only sent with timetamps aligned at a 10ms
boundary which was just wrong, now the buffer timestamp represents the
real time of the MIDI event.
Conveniently, the ALSA sequencer API supports scheduling events in the
future so the sequencer infrastructure can be used to have the tick
delivered at the right time, avoiding any custom scheduling mechanism.
The ticks scheduling starts on the first transition to PLAYING, and the
delay is also calculated when the pipeline goes into PLAYING.
https://bugzilla.gnome.org/show_bug.cgi?id=787683
When setting the "ports" property the value is duplicated but it's not
freed when the elements stops.
Reported by Valgrind (example run with "alsamidisrc ports=128:0"):
6 bytes in 1 blocks are definitely lost in loss record 30 of 1,911
at 0x4C2BBEF: malloc (in /usr/lib/valgrind/vgpreload_memcheck-amd64-linux.so)
by 0x5411528: g_malloc (gmem.c:94)
by 0x542A9FE: g_strdup (gstrfuncs.c:363)
by 0x775211E: gst_alsa_midi_src_set_property (gstalsamidisrc.c:284)
by 0x5184A4D: object_set_property (gobject.c:1439)
by 0x5184A4D: g_object_setv (gobject.c:2245)
by 0x51859DD: g_object_set_property (gobject.c:2529)
by 0x4F0474C: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
by 0x4F065C8: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
by 0x4F07557: ??? (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
by 0x4EFE3EE: gst_parse_launch_full (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
by 0x4EFE673: gst_parse_launchv_full (in /usr/lib/x86_64-linux-gnu/libgstreamer-1.0.so.0.1203.0)
https://bugzilla.gnome.org/show_bug.cgi?id=787683
The alsamidisrc element allows to get input event from ALSA MIDI
sequencer devices, and possibly convert them to sound using some
downstream element like fluiddec.
Fixes#738687
