iOS disables this feature under the hood, but it also distorts the images – unlike other mobile devices that don't support it. So this adds a check for both `background-attachment: fixed` support or whether it’s an iOS device - passing it adds the class `background-fixed` which is used to add the proper styles.
It also lowers the media query so the parallax-like style is present on a wider range of screens since this bug can be better targeted and avoided. In this way, screens that aren't the offending devices aren't punished merely based on screen size.
Props joemcgill, laurelfulford, helen.
Fixes#38395.
git-svn-id: https://develop.svn.wordpress.org/trunk@39297 602fd350-edb4-49c9-b593-d223f7449a82
05b394ccaa