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.

plugins/linux-v4l2/v4l2-input.c

@@ -912,11 +912,8 @@ fail:
 static void v4l2_update_source_flags(struct v4l2_data *data,
 		obs_data_t *settings)
 {
-	uint32_t flags = obs_source_get_flags(data->source);
-	flags = (obs_data_get_bool(settings, "buffering"))
-			? flags & ~OBS_SOURCE_FLAG_UNBUFFERED
-			: flags | OBS_SOURCE_FLAG_UNBUFFERED;
-	obs_source_set_flags(data->source, flags);
+	obs_source_set_async_unbuffered(data->source,
+			!obs_data_get_bool(settings, "buffering"));
 }
 
 /**