Rearrange vertex components

The first 16 component types are now reserved for the library.  This
leaves room for adding standard generic attributes, such as TBN vectors.
To make further room, texcoords were limited to four indices.

Generic attributes are limited to 24.  Current implementations seem to
only support 16 (the minimum allowed by the spec), so this is not a huge
problem.  There's also exactly enough room for another set of 24 generic
integer attributes.