clip-path: <geometry-box> examples (spec)

These work in Firefox and Safari. crbug.com/694218 tracks Chromium's implementation.

clip-path: none

hello world

clip-path: circle(50%)

hello world

clip-path: circle(50%) margin-box

hello world

clip-path: circle(50%) border-box

hello world

clip-path: circle(50%) padding-box

hello world

clip-path: circle(50%) content-box

hello world