fix muticompile for motion blur without alpha

Author: pmavridis

com.unity.render-pipelines.high-definition/Editor/Compositor/CompositorWindow.cs

@@ -150,7 +150,7 @@ void MarkShaderAsDirty(Shader shader, object context)
         private void OnDestroy()
         {
             CompositionManager compositor = CompositionManager.GetInstance();
-            if (compositor.shader != null)
+            if (compositor && compositor.shader != null)
             {
                 GraphData.onSaveGraph -= MarkShaderAsDirty;
             }

com.unity.render-pipelines.high-definition/Runtime/PostProcessing/PostProcessSystem.cs

@@ -1815,6 +1815,7 @@ void DoMotionBlur(CommandBuffer cmd, HDCamera camera, RTHandle source, RTHandle
             using (new ProfilingScope(cmd, ProfilingSampler.Get(HDProfileId.MotionBlurKernel)))
             {
                 cs = m_Resources.shaders.motionBlurCS;
+                cs.shaderKeywords = null;
 
                 CoreUtils.SetKeyword(cs, "ENABLE_ALPHA", m_EnableAlpha);
                 kernel = cs.FindKernel("MotionBlurCS");

com.unity.render-pipelines.high-definition/Runtime/PostProcessing/Shaders/MotionBlur.compute

@@ -5,7 +5,7 @@
 
 //#pragma enable_d3d11_debug_symbols
 
-#pragma multi_compile ENABLE_ALPHA
+#pragma multi_compile _ ENABLE_ALPHA
 
 RW_TEXTURE2D_X(CTYPE, _OutputTexture);
 TEXTURE2D_X(_InputTexture);