[VFX][Fix 1414185] Reset needsComputeBounds on switching boundsMode

This PR fixes an issue where the `needsComputeBounds` setting was not reset to false when switching `boundsMode `from Automatic to Recorded or Manual, leading to unused computations.
It also adds a warning about the cullings flags being changed to 
Fogbugz : https://fogbugz.unity3d.com/f/cases/1414185/

Packages/com.unity.visualeffectgraph/Editor/Data/VFXDataParticle.cs

@@ -276,6 +276,8 @@ public override void OnSettingModified(VFXSetting setting)
                     var graph = GetGraph();
                     graph.visualEffectResource.cullingFlags = VFXCullingFlags.CullNone;
                 }
+                else
+                    needsComputeBounds = false;
             }
             if (hasStrip)
             {
@@ -1382,5 +1384,17 @@ public void GenerateSystemUniformMapper(VFXExpressionGraph graph)
             m_GraphValuesLayout.SetUniformBlocks(orderedUniforms);
             m_GraphValuesLayout.SetPaddedSize(m_SystemUniformMapper);
         }
+
+        protected override void GenerateErrors(VFXInvalidateErrorReporter manager)
+        {
+            base.GenerateErrors(manager);
+
+            if (boundsMode == BoundsSettingMode.Automatic)
+            {
+                if (CanBeCompiled())
+                    manager.RegisterError("WarningAutomaticBoundsFlagChange", VFXErrorType.Warning,
+                        $"Changing the bounds mode to Automatic modifies the Culling Flags on the Visual Effect Asset to Always recompute bounds and simulate.");
+            }
+        }
     }
 }