i965/aub: Dump a final bitmap from DestroyContext.

i965/aub: Dump a final bitmap from DestroyContext.

Certain applications don't call SwapBuffers before exiting.  Yet, we'd
really like to see a bitmap containing the final rendered image even if
they choose never to present it.

In particular, Piglit tests (at least with -auto -fbo) fall into this
category.  Many of them failed to dump any images at all.

Dumping one final image at context destruction time seems to work.
We may wish to pursue a more elegant solution later.

Signed-off-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Eric Anholt <eric@anholt.net>