Fix delays in the end of Instant Preview rendering stroke

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 Krita will suspend the uploading process
   if the user desides to paint further instead of waiting for the
   background rendering to complete. This is the main part of this
   patch.

2) On resuming Krita will not upload the entire image to the canvas,
   but only a changed part. This is achieved by collecting dirty
   requests in KisImage::enableUIUpdates(). This method itself doesn't
   solve the initial problem, but it makes uploading a bit more efficient.

3) While the resume stroke is suspended, KisOpenGLCanvas2 blocks all the
   synchronizations of tiles' mipmaps. It means that normal lodN strokes
   will run with the mipmaps blocked. It is a bit dangerous approach, but
   it works until KisSuspendProjectionUpdatesStrokeStrategy is the only
   user of sigRequestLodPlanesSyncBlocked() signal.

CC:kimageshop@kde.org
BUG:361448
Fixes T2145