Defer loading off-screen pages. The current behavior preloads off-screen synchronously before displaying the current page incurring unnecessary latency.
Merge pull request #564 from achuang0/master

[pagingscrollview] Defer loading off-screen pages.
[core] Update NIIsRetina to handle higher screen scales.
Merge pull request #568 from jverkoey/retina

[core] Update NIIsRetina to handle higher screen scales.