[css-backgrounds] Add formula for continuous shift from sharp to rounded spread corners based on experimenting at TPAC with Bert, dbaron, plinss, and leaverou.
This not only results in continuous behavior, but gives better-looking results when the border radius is smaller than the spread radius.
Special thanks to dbaron for suggesting we create such a formula, to leaverou for creating an experimentation framework and doggedly searching for better formulas, and to plinss for the suggestion to try the cubic.
See spread-radius.html