In gegl_buffer_set_color_from_pixel(), if the pixel data, after
conversion to the buffer format, is fully zeroed, use clones of the
global empty tile for filling whole tiles, instead of creating a
new 0-filled tile.  This allows us to share memory with the global
empty tile, and makes uncloning the affected tiles more efficient.

Note that we deliberately don't just clear the corresponding area
of the buffer in this case, and make sure that there are actual
empty tiles in the requested area, to stay on the safe side (mostly
w.r.t. user-provided tile handlers, which may create non-zero tiles
when the underlying handler returns a NULL tile).