This is a derived class of HT_Physical_Renderer. Objects
of this class are renderers associated with hardware device drivers. They are
the last renderer sink in a renderer stack. The application is generally
required to have a variable of this class to reference the driver, but it
typically refers to the members of HT_Renderer.
Driver writers must derive a class from
HT_Device_Data. This class is the actual structure to which
the smart pointer HT_Device is pointing for the specific device
requirements.