Rearrange vertex attributes

Since fixed function is no longer supported, I don't have to care what
attribute number Nvidia gives the fixed function attributes.  The old
layout had weird holes and made custom attributes hard to add; now the
free attributes are neatly at the end.

The duality between standard and generic attributes has been removed.
User-defined attributes now have their own range.  No deprecated
versions have been kept since they would be difficult to support
properly.