Bug 981856. Use an 888 config when creating a pbuffer. r=jgilbert

Old versions of llvmpipe (8.0.4) seem to give out 565 configs that it can't
actually make PBuffers for. Explicilty ask for 8,8,8 to avoid that problem.

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extra : rebase_source : 2a667cc002b5ff4ad279e9483701eaf7952aeda8