Fix part of #33132: render resoltuion refresh issues after persistent images

was added for cycles.

This fixes the case where the option is disabled. I moved the option now to
Blender itself and made it keep the engine around only when it's enabled. Also
fixes case where there could be issues when switching to another renderer.

source/blender/render/intern/source/external_engine.c

@@ -382,6 +382,11 @@ int RE_engine_render(Render *re, int do_all)
 	if (type->render)
 		type->render(engine, re->scene);
 
+	if(!(re->r.mode & R_PERSISTENT_DATA)) {
+		RE_engine_free(re->engine);
+		re->engine = NULL;
+	}
+
 	if (re->result->do_exr_tile) {
 		BLI_rw_mutex_lock(&re->resultmutex, THREAD_LOCK_WRITE);
 		render_result_exr_file_end(re);

source/blender/render/intern/source/pipeline.c

@@ -244,6 +244,7 @@ Render *RE_GetRender(const char *name)
 	return re;
 }
 
+
 /* if you want to know exactly what has been done */
 RenderResult *RE_AcquireResultRead(Render *re)
 {
@@ -424,6 +425,19 @@ void RE_FreeAllRenderResults(void)
 	}
 }
 
+void RE_FreePersistentData()
+{
+	Render *re;
+
+	/* render engines can be kept around for quick re-render, this clears all */
+	for (re = RenderGlobal.renderlist.first; re; re = re->next) {
+		if (re->engine) {
+			RE_engine_free(re);
+			re->engine = NULL;
+		}
+	}
+}
+
 /* ********* initialize state ******** */