update whats new

dist/preview release/what's new.md

@@ -115,3 +115,4 @@
 - VertexData.merge no longer supports merging of data that do not have the same set of attributes. ([bghgary](https://github.com/bghgary)]
 - glTF 2.0 loader now creates a mesh for each primitive instead of merging the primitives together into one mesh. If a mesh only has one primitive, the behavior is the same as before. This change only affects meshes that have multiple primitives. ([bghgary](https://github.com/bghgary)]
 - Engine's onCanvasPointerOutObservable will now return a PointerEvent instead of the Engine. ([trevordev](https://github.com/trevordev))
+- Removed public references to default rendering pipeline's internal post process ([trevordev](https://github.com/trevordev))