fix some issues with the preview image: Preview generation is disabled by default and only done when requested

Cleared stale generation state at job start/end.
Fixed the animated preview GIF buffer leak.

expose.cpp

@@ -221,6 +221,10 @@ extern "C"
     {
         return sdtype_get_ongoing_generation_info();
     }
+    void sd_request_ongoing_generation_preview()
+    {
+        sdtype_request_ongoing_generation_preview();
+    }
 
     bool whisper_load_model(const whisper_load_model_inputs inputs)
     {