The problem was not related to the openGL actualy. It was caused
by the implementation of QPainter::clipRegion() which generated
20k+ rectangles from a rotated rect when rotation angle was near to
a 0 or 90deg.

WARNING for the future:
Never use QPainter::clipRegion(). Use KisPaintingUtils::safeClipRegion()
instead. It workarounds the issue returning a bit bigger rect.

Fixes T1446
BUG:355104
CC:kimageshop@kde.org