vbo: Implement method to track the inputs array.

vbo: Implement method to track the inputs array.

Provided the _DrawVAO and the derived state that is maintained if we have
the _DrawVAO set, implement a method to incrementally update the array of
gl_vertex_array input pointers.

v2: Add some more comments.
    Rename _vbo_array_init to _vbo_init_inputs.
    Rename vbo_context::arrays to vbo_context::draw_arrays.

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