The Input class is the parent class for all the Input modules
in the system, and so it is conceptually abstract -- it defines its destructor,
the getImage() and getDimension() methods as virtual. However,
it provides a minimal null implementation of an Input module, which
always returns NULL from getImage() and always returns a dimension
of pixels from getDimension().
This module is the default when no input module is specified, has the valid name of default, and takes no parameters.