Avoid potential visual glitch when switching between views and restoring previous state

[Federico M Pouzols]

The way #11708 is probably going to be addressed is not that great in the sense that in MdViewerWidget::switchViews() the views are rendered and after that the previous state of the view is restored (when switching back to a view that was previously manipulated). This can produces a visual glitch and a bit of delay.

Goal: avoid the glitch. This requires reorganizing ViewBase and derived classes so that they don't resetCamera(), resetDisplay() etc. in so many places, especially when doing render() and onRenderDone() and similar places. That is, at the very minimum it should be possible to render() VSI views using a reloaded/restored status.

I'm adding this tentative ticket for now, to add a comment and link to it in the code.

[Stuart Campbell]

This ticket has been transferred to github issue ​12577