Read plane

While working on ROI tool, noticed that the pixels store was not recycled. This was slowing down the retrieval of planes.  see https://trello.com/c/hZNSNO3h/53-bug-roi-5-0-3
Further improvement might happen, depending on time but not in this PR.

to test:
 * Draw a shape.
 * Go the graph pane.
 * This should work as before.