mesa: separate shader program object from shader object.

mesa: separate shader program object from shader object.

Currently a callback delete_shader_cb is used for deleting shader
and shader program objects. Mesa detaches all attached shaders in
_mesa_free_shader_program_data when deleting shader program objects. However
it is likely that these shaders have been freed in _mesa_free_shader,
which will result in unexpected behaviour. This fix uses a single callback for
shader program objects and deletes shader program objects before shader objects.