Alok's patch for 60719 ran into an issue where image layers are now going through the same path as content layers and are generating a paint rect that is larger than the image size (because the prepare step extends the paint rect to tile boundaries).
It's clear to me that the tiler shouldn't just grow its layer size incrementally. Root, content, and image layers all know their sizes explicitly. It seems clearer to just set this ahead of time and clamp edge tile sizes (and thus paint rects) to the layer size.
After some thought, I'm not sure this is a useful change. See comments on 60719 for more details.