use forward declaration to replace type cast

Change: 
   1.  use forward declaration to replace type casting,  clean related code. 
   2.  Potential issue fix:  in function CWelsLib::CreateIface,  it does not make sense to check ppEncCtx when decide to return 0 or 1.  

Review: 
https://rbcommons.com/s/OpenH264/r/81/