Avoid re-rendering when list item is spliced

[n1vlac]

If a single item is spliced, iron-list re-uses the spliced item's DOM elements to re-render the item below it, causing a downstream re-render of the remaining items. If the template contains an image, this can cause a visual flicker as the image elements are re-bound with new src values.

For example:

1. List of A B C D
2. Remove item C
3. List updates - A B (C -> D) (D -> hidden)

• DOM elements for C are re-used to render D
• DOM elements for D prior to deleting C are hidden

Ideally, D would not be re-rendered using C's DOM elements and simply remove/hide C.

An example use case is a user removing a video from a playlist, with each item in the list containing an image and text.