Move downscale functions out of tile-handler-zoom
Pass bpp instead of component count for boxfilter
Enable high quality scale for double precision formats
Get rid of function pointer in boxfilter

With the current logic it's an unnecessary complication.
operations: Remove calls to gegl_buffer_flush

These calls existed to cover some old bugs in Gimp's tile
handling but are now just causing unnecessary swap tile
writes. (If anything breaks because of this change that
code needs to explicitly call buffer_flush itself, instead
of relying on the operations to do it.)