Visual tearing on background grid

[mjsutcliffe99]

I'm not sure if this has already been reported, but I sometimes find the background grid gets all janky with some strange tearing going on, as pictured, when scrolling beyond frame. It's very superficial and not a big deal (zooming in and out seems to be an easy quick fix).

[RazinShaikh]

This might be due to caching the background but I have not verified this.
https://github.com/zxcalc/zxlive/blob/master/zxlive%2Fgraphview.py#L88

[jvdwetering]

I've noticed this mostly happens when you do a combined horizontal and vertical scroll using for instance your laptop touchpad. If you scroll regularly this seems to happen not at all or at least way less.