Re-fix a memory leak.

When we replace the ImageCanvas, the old canvas is still present in memory
because of the CanvasPaintPending system, so we have to make certain that
nobody is waiting for CanvasPaintPending for that canvas any more.

git-svn-id: https://valelab.ucsf.edu/svn/micromanager2/trunk@14443 d0ab736e-dc22-4aeb-8dc9-08def0aa14fd