Merge pull request #367 from UnityTech/on_demand_rendering

Automatic adjust on demand rendering.

Runtime/ui/window.cs

 using Unity.UIWidgets.editor;
 using Unity.UIWidgets.foundation;
 using UnityEngine;
+using UnityEngine.Rendering;
 
 namespace Unity.UIWidgets.ui {
     public delegate void VoidCallback();
 
         public const int defaultMaxTargetFrameRate = 60;
         public const int defaultMinTargetFrameRate = 25;
+        public const int defaultMaxRenderFrameInterval = 100;
+        public const int defaultMinRenderFrameInterval = 1;
 
         static Action _onFrameRateSpeedUp = defaultFrameRateSpeedUp;
 
         }
 
         static void defaultFrameRateSpeedUp() {
+#if UNITY_2019_3_OR_NEWER
+            OnDemandRendering.renderFrameInterval = defaultMinRenderFrameInterval;
+#else
+#endif
         }
 
         static Action _onFrameRateCoolDown = defaultFrameRateCoolDown;
         }
 
         static void defaultFrameRateCoolDown() {
+#if UNITY_2019_3_OR_NEWER
+            OnDemandRendering.renderFrameInterval = defaultMaxRenderFrameInterval;
+#else
+#endif
         }
     }
 }