Merge dirty_pcache_area_coords and zeroing_coords. Also interleave zeroing and updating pcache_area tasks. To zeroing a cached coord, pcache_area of descendants cached coords must be updated before zeroing. But, updating pcache_area of a cached coord must be performed after zeroing the coord. So, they are interleaved. dirty_pcache_area_coords are removed from redraw_man_t. It is merged to zeroing_coords.