Currently layout builds a new layer tree for every paint operation. To reduce the cost of drawing, to enable off-main-thread rendering, to reduce churn of graphics resources, and to enable layer-based scrolling, we need to retain the contents of layers between paints as much as possible.

Marking blocking 1.9.3 beta 1.

Layers repo: http://hg.mozilla.org/users/rocallahan_mozilla.com/layers-patches/