This should be filed under component "Drivers/Gallium/nouveau", but that
doesn't exist for some reason.

In the nv50 driver, after loading a map in the Left 4 Dead 2 beta,  there are
major rendering artifacts that make the game unplayable.  Portal shows a
similar problem.

Noticing that stable versions before Mesa 9.1 don't have this problem, I
bisected the regression to this commit:

http://cgit.freedesktop.org/mesa/mesa/commit/?h=48a45ec24ae74c0

commit 48a45ec24ae74c00d1487552e94d9f824a428f58
Author: Christoph Bumiller <e0425955@student.tuwien.ac.at>
Date:   Tue Jan 8 16:13:11 2013 +0100

    nouveau: improve buffer transfers

    Save double memcpy on uploads to VRAM in most cases.
    Properly handle FLUSH_EXPLICIT.
    Reallocate on DISCARD_WHOLE_RESOURCE to avoid sync.

I don't understand the changes made by this commit well enough to fix the
regression myself.  I have an apitrace that can be used to reproduce the issue,
and will comment on this bug with a link to it as soon as I find a good way to
share large (~512 MB) files online.