With 10.4.3 gpu compositing on chromium would result in a black window
on older Intel parts (ie x60s with 945gm) and at least some discrete
Radeon parts. These problems do not seem to occur on recent Intel
parts, though those have coherent memory and hardware contexts.
It isn't clear what changes in Mesa are involved in the problem
though it also occurs with the 10.3 branch.
