mesa: Make sure _TexEnvProgram points at the current ff fragment program

mesa: Make sure _TexEnvProgram points at the current ff fragment program

At least one place, the _mesa_need_secondary_color function in
state.h, uses this to make decisions.  The next patch in this series
will add another dependency.  Ideally, this field would go away and be
replace by a flag or something.

NOTE: This is a candidate for the 8.0 branch.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Eric Anholt <eric@anholt.net>