Only blur one quadrant of a box-shadow and mirror it to the other quadrants.
This applies only if the corners are symmetrical (square corners or equal
corner radii) otherwise we'll fall back to the old method.
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.
