textview: invalidate pixelcache for some invisible changes

When making changes above the current visible region, we might need to invalidate the pixelcache as the Y positions will no longer match. This usually is not needed because changes are made interactively and are made onscreen.

Other cases, though, can include an application changing the first line of the buffer automatically. We lose the ability to pixelcache well in this scenario, but that is unlikely an issue since rapid Y geometry resize or scrolling is less likely to be occuring. For situations where this is an issue, you can avoid removing the \n from the buffer so line heights are uneffected.

Fixes #2882 (closed)