libobs: Add ability to reroute encoders

Allows the ability for one encoder to defer to another in case of
failure or unsupported feature.  Okay, fine, it's mostly a hack so the
new NVENC encoder can fall back to the FFmpeg encoder if NV12 textures
aren't in use, that way it does not have to implement raw fallback
support itself.  The settings and properties are pretty much the same,
so there's no reason not to utilize it in order to save time that could
otherwise be spent more productively.

libobs/obs.h

@@ -1935,6 +1935,9 @@ EXPORT void obs_encoder_packet_ref(struct encoder_packet *dst,
 		struct encoder_packet *src);
 EXPORT void obs_encoder_packet_release(struct encoder_packet *packet);
 
+EXPORT void *obs_encoder_create_rerouted(obs_encoder_t *encoder,
+		const char *reroute_id);
+
 
 /* ------------------------------------------------------------------------- */
 /* Stream Services */