NEVERHOOD: Fix memory leak in BaseSurface

I hope this will help the memory leak reported as part of bug #6513
but my computer isn't quite up to the task of doing any extensive
Valgrind testing.

engines/neverhood/graphics.cpp

@@ -47,6 +47,7 @@ BaseSurface::BaseSurface(NeverhoodEngine *vm, int priority, int16 width, int16 h
 }
 
 BaseSurface::~BaseSurface() {
+	_surface->free();
 	delete _surface;
 }