Added a sinewave audio test source

- Added a test audio sinewave test source that should just play a sine
   wave of the middle C note.  Using unsigned 8 bit mono to test
   ffmpeg's audio resampler, seems to work pretty good.

 - Fixed a boolean trap in threading.h for the event_init function, it
   now uses enum event_type, which can be EVENT_TYPE_MANUAL or
   EVENT_TYPE_AUTO, to specify whether the event is automatically reset
   or not.

 - Changed display names of test sources to something a little less
   vague.

 - Removed te whole "if timestamp is 0 just use current system time"
   when outputting source audio, if you want to use system time you
   should just use system time yourself.  Using 0 as some sort of
   "indicator" like that just makes things confusing, and prevents you
   from legitimately using 0 as a timestamp for your audio data.

test/test-input/test-random.c

@@ -3,7 +3,7 @@
 
 const char *random_getname(const char *locale)
 {
-	return "Random Source";
+	return "20x20 Random Pixel Texture Source (Test)";
 }
 
 struct random_tex *random_create(const char *settings, obs_source_t source)