Use macros to improve safety loading callbacks

Just a minor fix mostly because I noticed that I kept accidentally
forgetting to add checks to the code properly.  This is one of those
cases where macros come in useful, as macros can automate the process
and help prevent these mistakes from happening by accident.

libobs/obs.h

@@ -377,8 +377,8 @@ EXPORT uint32_t obs_source_get_output_flags(obs_source_t source);
 /** Specifies whether the source can be configured */
 EXPORT bool obs_source_hasconfig(obs_source_t source);
 
-/** Opens a configuration panel and attaches it to the parent window */
-EXPORT void obs_source_config(obs_source_t source, void *parent);
+/** Updates settings for this source */
+EXPORT void obs_source_update(obs_source_t source, const char *settings);
 
 /** Renders a video source. */
 EXPORT void obs_source_video_render(obs_source_t source);