vbo: Walk the VAO to see if all varyings are in vbos.

vbo: Walk the VAO to see if all varyings are in vbos.

In vbo_draw_transform_feedback we currently look at
exec->array.inputs to determine if all varying
vertex attributes reside in vbos. But the vbo_bind_arrays
call only happens past the vbo_all_varyings_in_vbos
query. Thus we may work on a stale set of client arrays.
Using the current VAOs content for this query feels much
more logical to me.
Additionally with this change mesa makes more use of the
information already tracked in the VAO instead of looping
across VERT_ATTRIB_MAX vertex arrays.

Signed-off-by: Mathias Fröhlich <Mathias.Froehlich@web.de>
Reviewed-by: Eric Anholt <eric@anholt.net>