Refactor CWelsPreProcess

Change: 
  1.  Move pSpatialPic, uiSpatialLayersInTemporal, uiSpatialPicNum from sWelsEncCtx to CWelsPreProcess. 
  2.  Move/add related manage function to CWelsPreProcess class. 
  3.  Potential issue fix:  return success when Ctx->pVpp is alloced fail.  

  The reason of the change: 
    the pSpatialPic is picture list when store the src pictures.   It is need for pre-processing algorithms, and it is also input picture for encoding procedure.  
    I think it is more reasonable to maintain it in the CWelsPreProcess class,  it also can reduce complexity of sWelsEncCtx and make it more independent between CWelsPreProcess and real encoding procedure.   

Review:
https://rbcommons.com/s/OpenH264/r/83/