Several sweeping changes to graphics and image types. Graphics types

Several sweeping changes to graphics and image types.  Graphics types
now have their names associated with them so that code can dispatch on
the name.  Each image type is now associated with a particular
graphics type, so that it is possible to get the image type given a
graphics type.  New procedures GRAPHICS-TYPE and IMAGE-TYPE provide
very general ways to get pointers to such types.  New procedure
GRAPHICS-TYPE-NAME gets the name of a graphics type.

These changes necessitated some changes in the interface to the
graphics type definitions.  In particular, there's no longer a
procedure to register a graphics type's name, and also the procedure
to construct a graphics type now accepts an additional argument, which
is the name.