constify WIP

error C2259: 'CAviSplitterFilter' : cannot instantiate abstract class   H:\progs\Compiling\mpc-hc_dev\src\DSUtil\DSUtil.h   278 1   mpc-hc
error C2259: 'CMpegSplitterFilter' : cannot instantiate abstract class  H:\progs\Compiling\mpc-hc_dev\src\DSUtil\DSUtil.h   278 1   mpc-hc
error C2259: 'CAviSourceFilter' : cannot instantiate abstract class     h:\progs\compiling\mpc-hc_dev\src\mpc-hc\FGFilter.  97  1   mpc-hc