rrdtool: queue post_draw_cmd to be after images

rrdtool: queue post_draw_cmd to be after images

The purpose of the post_draw_cmd is to execute something after the
rrdtool has drawn the images. Right now it executes the command
actually before drawing the images, as the command gets executed from
the high priority work that queued the image drawings.

This is not what we want. Therefore queue the post_draw_cmd to the
same low priority queue where the images are put. This guarantees that
the command does not begin too soon. On uniprocessors it is also
guaranteed that the command does not start executing before all of the
images are completed.

Signed-off-by: Timo Kokkonen <timo.t.kokkonen@iki.fi>