OpenGL benden lines are almost gone

[Dmitry Kazakov]

Hi!

I've just pushed a workaround for the bended lines in OpenGL into master.
Now they are amost gone! And you can test it =)

Actually, they still bend a bit, but only when you paint really huge
circles (> 1000px in diameter) with very small brushes (~5px). For
everything smaller, I couldn't see any difference with QPainter canvas. See
a screenshot [0]. The bottom stroke is actually about 1000px in height.

And I need a bit of your input about this patch. Actually, it has two
approaches to fixing the problem: first it tries to disable Double
Buffering, then, if it fails, it tries to disable VSync using pure OpenGL
calls. First approach gives a bit better results. I need to find out
whether there are any system/cards which do not allow disabling Double
Buffering (probably, openGL ES systems (?)).

So if you see a message in Krita terminal saying:

"WARNING: Your system/Qt/video adapter doesn't support single buffered
contexts. Trying to workaround it by disabling VSync."

just let me know about it =)

[0] - http://susepaste.org/70034032

-- 
Dmitry Kazakov
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