multicam: refactor to heavily optimize multicam monitor retrieval

app/render/rendermanager.h

@@ -117,6 +117,7 @@ public:
       force_size = QSize(0, 0);
       force_channel_count = 0;
       mode = m;
+      multicam = nullptr;
     }
 
     void AddCache(FrameHashCache *cache)
@@ -134,6 +135,7 @@ public:
     bool use_cache;
     ReturnType return_type;
     RenderMode::Mode mode;
+    MultiCamNode *multicam;
 
     QString cache_dir;
     rational cache_timebase;