Followup to bug 604533. We only need to support up to http://mxr.mozilla.org/mozilla-central/source/gfx/layers/ThebesLayerBuffer.cpp#288, which could boil away to a single memmove() if the cairo analysis were smart enough. Matt, Rob told me you started on something in this area and might have some code already.

Suggest blocking fennec-final, because it's an optimization transparent to gecko code that should be a big win for scrolling perf, when that's tuned.