[css-images] Wrong definition for `object-position` percentages

[Loirooriol]

https://drafts.csswg.org/css-images-3/#the-object-position

Percentages: refer to width and height of element itself

That's wrong. It should be analogous to background-positition:

Percentages: refer to size of background positioning area minus size of background image; see text

A percentage for the horizontal offset is relative to (width of background positioning area - width of background image). A percentage for the vertical offset is relative to (height of background positioning area - height of background image), where the size of the image is the size given by background-size.

Otherwise, the initial object-position: 50% 50% wouldn't center the image!