Hourglass while post-processing change being processed

[perdrix52]

When DSS is making a change to brightness or saturation etc on a finished stack, it progresses in a spiral tile-by-tile fashion.
Sometimes when the changes are subtle it's hard to see the progress of the tiles, so I wait for 20 seconds and then hope it's finished updating the image.
Hows about a boring progress bar or hourglass to show the progress ?

[SimonCSmith]

I've added a progress bar to show processing dialog progress along the bottom of the image frame. Note that this code is not be thread safe (outside of the main thread) because it relies on querying the state of the processing rects that could be reallocated on an Init() call if done on the non-main (GUI) thread.