Avoid 'blanking' screen when loading new content if possible

[cmdcolin]

Currently when the view is changed, the screen will be "blanked" (any features or renderings on screen go away, just a loading bar appears)

this reduces the continuity of user experience

If we could keep old rendered contents on screen as long as possible, with a clear loading bar over it, then the experience would be somewhat similar to UCSC

See UCSC screnshot here, you scroll a bit, and then it doesn't go blank but keeps old results visible with clear loading state

This issue is one of these things that is sort of related to challenges introduced by web worker RPC