backgroundTransition should also be inhibited during auto-animate

[devkev]

Slide transitions are disabled during auto-animate, which makes sense because the content is being auto-animated, not transitioned. However, any backgroundTransition is still applied between auto-animate slides, which is very noticeably wrong when using eg. slide.

Reproducer (live demo of problem, live demo of desired behaviour):

<style>
	.slide-background-content {
		background: url('https://placekitten.com/450/300');
		background-size: cover;
	}
</style>

...

<section>
	First Slide
</section>
<section data-auto-animate>
	<h1>Implicit</h1>
</section>
<section data-auto-animate>
	<h1>Implicit</h1>
	<h1>Animation</h1>
</section>
<section>
	Last Slide
</section>

...

<script>
	Reveal.initialize({
		transition: 'slide',
		backgroundTransition: 'slide',
	});
</script>