Required to have an alpha channel (which in the strange world of graphics seems to refer to transparency, and not experimental code) in WebM videos. Straight port of Firefox 53 implementation, adapted slightly for a couple of patches previously taken without this.
Ref: BZ 1320829
Since these are just interpreted comments, there's 0 impact on actual code.
This removes all lines that match /* vim: set(.*)tw=80: */ with S&R -- there are
a few others scattered around which will be removed manually in a second part.
