meta: Fix saving the active program

meta: Fix saving the active program

When saving the active program in _mesa_meta_begin, it was actually
saving the fragment program instead. This means that if the
application binds a program that only has a vertex shader then when
the meta saved state is restored it will forget the bound program.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=41969
Reviewed-by: Chad Versace <chad@chad-versace.us>