css: Add support for conic-gradient()

css: Add support for conic-gradient()

This comes complete with animation support. For a good time, try:

@keyframes conic {
  100% { background-image: conic-gradient(from 1turn, red, lime, blue, yellow, red); }
}

window {
  background-image: conic-gradient(red, lime, blue, yellow, red);
  animation: conic infinite linear 5s;
}