Fix crash with display mode switch.

With slow display update it could happen that on
display mode switch the underlying C++ implementation
was already freed while signaling the view was
still in progress.

Protect with better check.

#RH-34776

RenderEngine.cs

@@ -60,6 +60,22 @@ public partial class RenderEngine : AsyncRenderContext
 		/// </summary>
 		public Client Client { get; set; }
 
+		/// <summary>
+		/// True when State.Rendering
+		/// </summary>
+		public bool IsRendering { get { return State == State.Rendering; } }
+		/// <summary>
+		/// True when State.Uploading
+		/// </summary>
+		public bool IsUploading { get { return State == State.Uploading; } }
+		/// <summary>
+		/// True when State.Waiting
+		/// </summary>
+		public bool IsWaiting { get { return State == State.Waiting; } }
+		/// <summary>
+		/// True when State.IsStopped
+		/// </summary>
+		public bool IsStopped {  get { return State == State.Stopped; } }
 		/// <summary>
 		/// Current render engine state.
 		/// </summary>

Viewport/RenderedViewport.cs

@@ -232,8 +232,11 @@ void m_cycles_PassRendered(object sender, ViewportRenderEngine.PassRenderedEvent
 			m_frame_available = true;
 			m_available = true;
 			m_started = true;
-			if(e.Sample <=1) SetView(e.View);
-			SignalRedraw();
+			if (m_cycles?.IsRendering ?? false)
+			{
+				if (e.Sample <= 1) SetView(e.View);
+				SignalRedraw();
+			}
 		}
 
 		void m_cycles_StartSynchronizing(object sender, EventArgs e)