Incorrect painting order for non-positioned floats

[kwkbtr]

Non-positioned floats should be painted below positioned elements with stack level 0. (cf. CSS 2 - Layered presentation) However, with Vivliostyle, when a non-positioned float comes after a positioned element with stack level 0, the float is painted above the positioned element, since Vivliostyle uses absolute positioning to emulate floats.

Example of a failing test: css21/floats-clear/float-006.xht