Feature request: ability to mark out-of-date tiles within JSObserver

[pironic]

This ticket is a copy of overviewer/Minecraft-Overviewer#804 and has been migrated to continue discussion on the possibility of an addon feature.

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@HTL2001 originally wrote:

The way I see this is an extension of JSObserver, where it could apply some sort of colorization over tiles that are pending re-render. File "progress.json" would contain a list of tiles to apply the color to. Not sure how feasible this is though...