Zero height cells cause strange layout issues

[JosephDuffy]

If a zero height cell is present all the visible cells after disappear. An example is available in the branch zero-height-cell-bug.

I'm not sure which project this is a bug in. I'd guess ComposedLayouts but I can't be sure.

I've done a search for 0 and height but can't find anything that would point to this.

Note that the cells are not requested and are not in the hierarchy. Maybe the position for them is wrong (e.g. off the screen) so the collection view never requests them.

Screenshots:

App launch

Scrolling down

Post-delete

cc @shaps80

[shaps80]

Hmm seems unlikely to be the layout since we use whateva UIKit gives us, I'll need to investigate deeper.