mesa: Provide an alternative to get_vp_mode()

mesa: Provide an alternative to get_vp_mode()

To get equivalent information than get_vp_mode(), track the vertex
processing mode in a per context variable at
gl_vertex_program_state::_VPMode.
This aims to replace get_vp_mode() as seen in the vbo module.
But instead of the get_vp_mode() implementation which only gives correct
answers past calling _mesa_update_state() this context variable is
immediately tracked when the vertex processing state is modified. The
correctness of this value is asserted on state validation.

With this in place we should be able to untangle the dependency with
varying_vp_inputs and state invalidation.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Brian Paul <brianp@vmware.com>