Fixed spot cookies.

ScriptableRenderPipeline/LightweightPipeline/LightweightPipelineUtils.cs

 
         public static bool IsSupportedCookieType(LightType lightType)
         {
-            return lightType == LightType.Directional;
+            return lightType == LightType.Directional || lightType == LightType.Spot;
         }
 
         public static bool PlatformSupportsMSAABackBuffer()

ScriptableRenderPipeline/LightweightPipeline/Shaders/LightweightLighting.cginc

         float2 cookieUV = mul(_WorldToLight, float4(worldPos, 1.0)).xy;
         return tex2D(_MainLightCookie, cookieUV).a;
     #elif defined(_MAIN_SPOT_LIGHT)
-        float3 projPos = mul(_WorldToLight, float4(worldPos, 1.0)).xyz;
+        float4 projPos = mul(_WorldToLight, float4(worldPos, 1.0));
         float2 cookieUV = projPos.xy / projPos.w + 0.5;
         return tex2D(_MainLightCookie, cookieUV).a;
     #endif // POINT LIGHT cookie not supported