libobs: Don't use source flags for async buffering

(This commit also modifies the decklink, linux-v4l2, mac-avcapture,
obs-ffmpeg, and win-dshow modules)

Originally, async buffering for sources was supposed to be a
user-controllable flag.  However, that turned out to be less than ideal
because sources (such as the win-dshow plugin) were programmed with
automatic control over their buffering (such as automatically detecting
USB 2.0 capture devices and then enabling in those cases).

The fact that it was a flag caused a design flaw to where buffering
values would be overwritten when a source is loaded from save data.

Because of that, this flag is being deprecated and replaced with a
specific function to enable unbuffered mode instead.

libobs/obs.h

@@ -834,8 +834,8 @@ EXPORT bool obs_source_active(const obs_source_t *source);
  */
 EXPORT bool obs_source_showing(const obs_source_t *source);
 
-/** Specifies that async video frames should be played as soon as possible */
-#define OBS_SOURCE_FLAG_UNBUFFERED             (1<<0)
+/** Unused flag */
+#define OBS_SOURCE_FLAG_UNUSED_1               (1<<0)
 /** Specifies to force audio to mono */
 #define OBS_SOURCE_FLAG_FORCE_MONO             (1<<1)
 
@@ -1100,6 +1100,10 @@ EXPORT uint64_t obs_source_get_audio_timestamp(const obs_source_t *source);
 EXPORT void obs_source_get_audio_mix(const obs_source_t *source,
 		struct obs_source_audio_mix *audio);
 
+EXPORT void obs_source_set_async_unbuffered(obs_source_t *source,
+		bool unbuffered);
+EXPORT bool obs_source_async_unbuffered(const obs_source_t *source);
+
 /* ------------------------------------------------------------------------- */
 /* Transition-specific functions */
 enum obs_transition_target {