Needs your testing!

Test Plan:

1) Paint with complex brushes on Float32 images
2) Try painting many repetitive strokes, there should be no
   flockering and no delays.
3) Try cancel your srokes before they are finished with Esc key.
   Please note that the most dangerous moment is when Krita writes
   "Updating..." right before background stroke calculation is
   finished. Try pressing exactly at this moment! :)

Technical Details:

There was a short delay in the end of every Instant Preview
regeneration stroke. It was mostly seen on bigger canvases and
Float32-bitdepth mode.

The cause of this delay was the "update resume" stroke that
tried to upload the 100%-zoom image data to the canvas. Sometimes
such uploading could take up to 1.5 seconds, which could interfere
into the painter's workflow.

This patch does multiple things to mitigate this problem:

1) KisSuspendProjectionUpdatesStrokeStrategy is now suspendable
   (again). It means that Krit...