Made PortalClipEngine extend DoomClipEngine. (Slight change of strategy)

I realized that the PortalClipEngine::slideMove would look pretty much
the same as DoomClipEngine::slideMove, same with the teleport move
functions, so I made PortalClipEngine extend DoomClipEngine and then
removed those method overrides. Seems to work just fine :) Doing this
will allow me to concentrate on the clipping code a feature at a time
rather than having a big broken thing that has to all be re-written.