Before this PR each tile stores its own dirty rect relative to the entire image. This is somewhat confusing because this field is used to track partial uploads correctly and uploads reason about dirty rects relative to the tiles so it has to do some conversions. Storing relative to the tile is also truer to the "device space" the rects are tagged with.
Last but not least the image-relative coordinates will make even less sense for tiled blob images as we start trating them as virtually infinite planes.