Remove -webkit-backface-visibility on skew mixin

Ref #5319 - This rule is causing pixelated edges on iOS. Removing
it makes everything smooth across all OS's, browsers, and devices.
fixes #7503: don't do dat
Merge pull request #7148 from retlehs/3.0.0-wip

Remove -webkit-backface-visibility on skew mixin
Merge remote-tracking branch 'upstream/3.0.0-wip' into 3.0.0-wip