Backout fa9c6845338e (bug 1085223) for causing bug 1087257.
Bug 1081072, part 2: add reftest.
Bug 1084500 part 1: Refactor ComputeBackgroundAnchorPoint to take a Position & use a per-component helper method. r=seth
Bug 1084500 part 2: Promote & rename ComputeBackgroundAnchorPoint to nsImageRenderer::ComputeObjectAnchorPoint. r=seth
Bug 1084500 part 3: Make documentation for ComputeObjectAnchorPoint cover non-background use cases. r=seth