The remove boost::shared_ptr and the rendering library branches where
developed simultaneously, and thus some of the rendering library
implementation was using boost::shared_ptr like the old code. Bring up
to date with the rest of VTK-m by using std::shared_ptr instead.
The clang compiler was running into linker errors constructors of
classes with virtual methods that were inline and destructors that were
not declared at all. In this case, the compiler was not creating
everything needed by a virtual table and the link died.
