Dynamic lazyrenderThreshold

[KeithM23]

Hi, thanks for the awesome library.

I just have a small issue, possibly a very small use case for this but I need the lazyrenderThreshold value to trigger each/the necessary pages _shouldPageRender methods.

I'm currently dynamically changing the lazyrenderThreshold value but unfortunately it's not causing the _shouldPageRender method on the pages to fire as the only thing that triggers this is onPageChange.

So for example if I have the lazyrenderThreshold set to 0, then i change it to 5, without changing the page(i.e. swiping etc..), no lazy rendering occurs. However when I do change the page the other pages start to lazy load as onPageChange triggers _shouldPageRender

Unfortunately I need this to happen without a page change so the above isn't okay for my use case. I tried to implement this in hopes of doing a PR but can't workout how to best implement it.

What I have works but is definitely a dirty/hacky way of doing this so don't want to do a PR with it.

I'm just calling _onMomentumScrollEnd (as it calls onPageChange on each of the pages) whenever the lazyrenderThreshold changes which works but isn't great.

Thanks again for the awesome library!