As of April 2012, both WebKit and Firefox support this but
in different ways - and different versions of Firefox implement it differently.
WebKit - seems to work as expected
Firefox 11 - origin is oddly positioned - 0,0 is
the centre. Also, doesn't refresh properly e.g.
doesn't appear to "clear the canvas" when
disabling 3D styling; doesn't redraw affected elements
when mozPerspectiveOrigin is changed
Firefox 14 - origin is same as WebKit, but zoomed
elements are clipped to the bounding box of their unzoomed