intel: Ensure renderbuffers are current when mapping them.

intel: Ensure renderbuffers are current when mapping them.

In the case of renering to windows in X, we would render to stale buffers
(or not render at all!) if you hit a MapRenderbuffer as the first thing
done to your window after new buffers are ready to be collected in DRI2.

I think this also covers the weird comment about irb->mt being missing
sometimes.

Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Reviewed-by: Brian Paul <brianp@vmware.com>