Fix viewport flickering when multi-frame rendering API is used with certain parameters

Our user was getting flickering when using accumulation motion blur with certain parameters:
https://jira.unity3d.com/browse/UUM-59115

The root cause of this was that we were not properly handling the case that the first sample in the Monte Carlo accumulation has zero weight. I've re-arranged the formula to output zero in this case. 

Path tracing without motion blur always uses a weight of one for all frames, so it should not be affected. @pieterjan-bartels if you think there is an issue let me know.
 
I've also fixed an error in the example script in the documentation: it was capturing a screenshot in two places, only one is needed.

Author: pmavridis

Packages/com.unity.render-pipelines.high-definition/Documentation~/rendering-combine-animation-sequences-in-script.md

@@ -84,14 +84,5 @@ public class FrameManager : MonoBehaviour
         // Make sure the shutter will begin closing sometime after it is fully open (and not before)
         shutterBeginsClosing = Mathf.Max(shutterFullyOpen, shutterBeginsClosing);
     }
-
-    void Update()
-    {
-        // Save a screenshot to disk when recording
-        if (m_Recording && m_Iteration % samples == 0)
-        {
-            ScreenCapture.CaptureScreenshot($"frame_{m_RecordedFrames++}.png");
-        }
-    }
 }
 ```

Packages/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Accumulation/Shaders/Accumulation.compute

@@ -73,11 +73,13 @@ void KMain(uint3 dispatchThreadId : SV_DispatchThreadID)
 #ifdef INPUT_FROM_FRAME_TEXTURE
         float4 color = _FrameTexture[COORD_TEXTURE2D_X(dispatchThreadId.xy)];
 #else
-        float4 color = _CameraColorTextureRW[COORD_TEXTURE2D_X(dispatchThreadId.xy)];
+        float4 color = _CameraColorTextureRW[COORD_TEXTURE2D_X(dispatchThreadId.xy)] * _AccumulationWeights.x;
 #endif
 
-        if (sampleCount++)
-            color = (_AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] * _AccumulationWeights.y + _AccumulationWeights.x * color) * _AccumulationWeights.z;
+        if (sampleCount > 0)
+            color = (_AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] * _AccumulationWeights.y + color);
+
+        color *= _AccumulationWeights.z;
 
         _AccumulatedFrameTexture[COORD_TEXTURE2D_X(currentPixelCoord)] = color;
 
@@ -86,7 +88,7 @@ void KMain(uint3 dispatchThreadId : SV_DispatchThreadID)
         color *= exposureMultiplier;
 
         // Add a little convergence cue to our result
-        AddConvergenceCue(currentPixelCoord, sampleCount, color.xyz);
+        AddConvergenceCue(currentPixelCoord, sampleCount + 1, color.xyz);
 
         _CameraColorTextureRW[COORD_TEXTURE2D_X(currentPixelCoord)] = color;
 #endif

Packages/com.unity.render-pipelines.high-definition/Runtime/RenderPipeline/Accumulation/SubFrameManager.cs

@@ -322,7 +322,8 @@ float ShutterProfile(float time)
             else if (time > m_ShutterBeginsClosing)
             {
                 float closingSlope = 1.0f / (1.0f - m_ShutterBeginsClosing);
-                return 1.0f - closingSlope * (time - m_ShutterBeginsClosing);
+                // We are using max to prevent the weight from going negative due to numerical imprecision 
+                return Mathf.Max(0.0f, 1.0f - closingSlope * (time - m_ShutterBeginsClosing));
             }
             else
             {