Support deletion of tweets

[tokee]

Creating a collage of millions of tweeted images takes days. Re-rendering the collage to honour deleted tweets could be done without re-downloading the images, but it would still take a lot of time.

Idea: Derive the IDs for the deleted tweets, calculate their position in the grid and replace the image-tiles with blanks, then update the affected tiles in the pyramid layer above and so forth up to the top. This would have the combined effect of being a lot faster than a full re-render (obvious plus) and to show how many images are retracted (plus/minus, depending of what one wants to show).

Optional: Instead of a blank, use a special image, such as a distinct note saying the image has been removed.