dri3: Add GLX_EXT_buffer_age support

dri3: Add GLX_EXT_buffer_age support

v2: Indent according to Mesa style, reuse sbc instead of making a new
    swap_count field, and actually get a usable back before returning the
    age of the back (fixing updated piglit tests).  Changes by anholt.

Signed-off-by: Adel Gadllah <adel.gadllah@gmail.com>
Reviewed-by: Robert Bragg <robert@sixbynine.org> (v1)
Reviewed-by: Adel Gadllah <adel.gadllah@gmail.com> (v2)
Reviewed-by: Eric Anholt <eric@anholt.net>