rasterStep

The raster step (#209, #308): its box is region under ctm in device pixels, cut to the clip. A render paints into a bitmap context of the box, as a soft mask's group does. The backdrop is a snapshot of the host when it is a bitmap context with no transparency layer open. Past MASK_PIXEL_BUDGET the step works at a lower resolution, and then reads no backdrop. A raster's first row is the top of the box. Quartz's base space runs up, but a context's device space can run either way, and a bitmap context's runs down, so the step reads which way from the context (#308).