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 can reduce complexity of sWelsEncCtx and make it more independent between CWelsPreProcess and real encoding procedure.   

Review:
https://rbcommons.com/s/OpenH264/r/83/