Fixed slow qgraphics on Linux.

[xraybrian]

This is less a bug report, but a suggested addition.

My code, based on nodegraphqt works great under windows, but the graphics were sluggish under Linux.

I was able to fix this issue by adding:

self._viewer.setViewportUpdateMode(QGraphicsView.BoundingRectViewportUpdate)

to the NodeGraph class init method.

I think there is perhaps a bug or issue on some systems regarding deciding on what to update.

Anyhow, this solved the issue entirely.

[xraybrian]

Just an update, although this fixes the slow graphics updating under Linux, it introduces some funny behavior when re-sizing backdrops. Only the corner being dragged gets updated until the backdrop is manually moved, or a zoom/pan is initiated.