Paint only visible items of list/grid canvas items. Add large list demo page.

[cmeyer]

• Paint only the visible rows of a list canvas item.
• Paint only the visible cells of a grid canvas item.
• Demonstrate lists and grids larger than their viewport in the ui demo.

[Ion-e]

What is the scope of this ticket? I have noticed performance overall in handling large number of items is better but could you elaborate a bit more on your point with specific reproduceable case of the original issue? Thanks.

[cmeyer]

These optimizations do not get used anywhere on the Swift side currently; they're cleanup to bring the older list/grid canvas items (superseded by grid flow canvas item) to the same level of optimization.