Efficient multithreading support for Krita

by Dmitry Kazakov for KDE

This project will make Krita merge process threaded. I will encapsulate all the update information into a single struct and make the merge process be executed on any of arbitrary number of threads in parallel. More than that, the synchronous part of the update signal to the UI thread will eliminate flickering on the canvas. The update scheduler will plan which rectangles of the image can be updated simultaneously and will optimize update requests queue so no memory conflicts will appear.