If many curves were drawn, image rendering can be slowly

[smay1613]

This issue is caused by antialiasing.
Firstly, antialiasing was an option.
But when FramebufferObject is set as QQuickItem rendering target, it can boost antialiasing - you can draw much more shapes before it will be slowed down.
So if lags start, user can disable antialiasing option.
But changing antialiasing option dynamically if render target is FrameBufferobject causes application crash - it is Qt bug.
See https://bugreports.qt.io/browse/QTBUG-46434.