Plot display progress indicator when Blend Modes enabled.

[led]

With larger images and renders having the Blend Mode set to anything other than 'Normal', it can take quite a long time for the plot to fully compose and display (ie a number of minutes). Sometimes it's difficult to know whether it's finished composing the plot or not.

It would be useful to have some indicator of the compositing progress, or at least something that it was still in-progress, and perhaps a notification toaster when it's completed.