Add missing observer trigger - onLoadedObservable notify method for hdrCubeTexture

[slash9494]

I found a minor issue during loading textures through add task of asset manager
when I try to load the same texture files several times for different purposes, cubeTextureTask can be finished normally but hdrCubeTextureTask can't be finished even though it has been already finished

• this playground can show task count on console
  https://playground.babylonjs.com/#M5L9FH

So, I have debugged and I found the creating function of hdrTexture has lost the load observable notify method that is included in the creating function of cubeTexture

• engine.cubeTexture.ts

• engine.rawTexture.ts

I add above code in raw texture and then it worked well

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