Don't have SEncParamExt inherit SEncParamBase

Instead just duplicate the common fields. These fields had to
be duplicated for the C interface compatibility anyway - but
this way there is no risk to accidentally introduce an ABI
break since there is no need for the layout of SEncParamBase to
actually match the start of SEncParamExt.

codec/encoder/plus/inc/welsEncoderExt.h

@@ -66,7 +66,7 @@ class CWelsH264SVCEncoder : public ISVCEncoder {
   /*
    * return: CM_RETURN: 0 - success; otherwise - failed;
    */
-  virtual int EXTAPI Initialize (const SEncParamBase* argv, const INIT_TYPE init_type);
+  virtual int EXTAPI Initialize (const void* argv, const INIT_TYPE init_type);
 
   virtual int EXTAPI Uninitialize();