New feature: only animate elements in viewport

[lesliecdubs]

Nice port of Rellax for React!

One feature I always wanted in Rellax was to limit animations to elements currently visible in the viewport. This is especially important for performance on long pages with lots of parallaxing elements.

I'll try to take a look at this myself if I can carve out the time, but thought I'd leave a note here too! Cheers.