Store compiled vertex program representations in a pointer in the

Store compiled vertex program representations in a pointer in the
vertex_program struct.

Allow switching between regular and vertex_program implementations
of fixed function TNL with the MESA_TNL_PROG environment var
(previously this required recompilation).

Ensure program compilation only references program data, not the
wider context.  This means that compiled programs only need to be
invalidated when the program string changes, not on other state
changes.