Compute VAO buffer binding information past the position/generic0 mapping.
Scan for duplicate buffer bindings and collapse them into derived
effective buffer binding index and effective attribute mask variables.
Provide a set of helper functions to access the distilled
information in the VAO. All of them prefixed with _mesa_draw_...
to indicate that they are meant to query draw information.

v2: Also group user space arrays containing interleaved arrays.
    Add _Eff*Offset to be copied on attribute and binding copy.
    Update comments.

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