[css-page-floats] The 'clear' values are backwards

The page-float 'clear' values (block-start/end, and ones that act like
that) are defined backwards - as written, they prevent *further*
page-floats in the same position.  That's not how clear works in other
contexts - 'clear' indicates that the element wants to move if *it*
isn't the first in that direction.

So "clear: block-start;" should be defined as making the page float in
question move to the next appropriate fragment if it's not the first
float in that position.

~TJ and fantasai