Bug 361923

https://share.kde.org/index.php/s/t83Vga0cIgr7gfg/download
[^ video of the issue, 50 second, *.webm , 5MB.  Quick description: first 25 second (left) 50% viewport all is fine, last 25 second (right) 44% viewport and bug ]

Hi, I open here a bug report about inconsistencies of rendering (brush stroke) depending viewport zoom: in 3.0(alpha) my brush strokes are all rendered correctly at discrete zoom of viewport, but  not when I use a random viewport zoom value. 

How to reproduce
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* Use a brush preset as 'Deevad E4 Splat'  ( http://www.peppercarrot.com/extras/resources/deevad.bundle )
* Be sure viewport zoom is at 50% ( you can use numkeypad+ and numkeypad- to navigate from discrete zoom to discrete zoom )
* paint strokes :  they render perfectly on release of stylus or mouse-button.
* Now, zoom to a value 'not discrete',  ( in video 44% ) with Ctrl+Space+drag 
* Paint stroke with the same preset

Result:
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After the release of stylus or mouse-button, the stroke is changing *totally*. It produce a non WYSIWYG experience of painting. (a bit like typing a letter on keyboard, and having another letter on screen). This is even more dramatic after painting a long stroke to add texture to a scene, and then release the stylus and get a totally different rendering.
Another effect is also something like 'self-collision' of stroke and tile flicker if you paint over another stroke. It's visible on the video. I describe it like a semi-effect of the brush blending mode 'copy'. 

References/Spec:
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- Intel(R) Core(TM) i7 CPU 870 @ 2.93GHz
- GeForce GTX 650 Ti , Nvidia proprietary driver 340update, OpenGL canvas 'on'
- Ubuntu 16.04, Cinnamon desktop 2.8. 
- Video recorded in mid of week ( git6b2491e ) , reproduced with a daily build of 1 min ago ( git fb07755 )