Merge 130322 - [Sub-pixel layout] incorrect rendering when painting sub-layers as their own root
Reviewed by Eric Seidel.
When in compositing mode, layer painting can be triggered through the backing store. When this
happens, a non-top-level RenderLayer is called to paint as its own root. Normally, we attempt to preserve
the proper sub-pixel accumulation through layers to their children, but since we're not
starting with the top-level layer, we haven't properly accumulated one, and convertToLayerCoords,
another source of correctly getting the sub-pixel offset for a layer, also avoids crawling
past the listed root layer.
When painting a root layer, we're aligned to the surface we're painting to, so we round our
offset to avoid moving objects around.